Need help with BYTES

first understand that a string and an array can not be used the same way.
you can not print an array of bytes and a string the same way or read them the same way. Arrays of bytes have to be read or printed one byte at a time unlike strings. You can access each byte of the string through the corresponding index and print its "byte value". if you want to see the actual character you have to convert the byte to a char such as:

name:string = "Jesse"
print name[0]
print chr(name[0])

this will print 74 and the letter "J"

with this in mind I have somewhat fixed your code to work first with bytes:

SuperStrict

Graphics 640, 480

Local client:OClient = OClient.Create("127.0.0.1", 6121)

While Not KeyHit(KEY_ESCAPE)
	client.onRecieve()
Wend

Rem
Function ToString(bytes:Byte[])
	Return string.bytestostring(bytes,bytes.length)
End Function
EndRem

Type OClient
	Field host:String
	Field port:Int
	Field socket:TSocket
	Field stream:TSocketstream
	Field buffer:TBytes
	
	Function Create:OClient(host:String, port:Int)
		Local client:OClient = New OClient
		client.host = host
		client.port = port
		client.socket = CreateTCPSocket()
		ConnectSocket(client.socket, HostIp(host), port)
		
		If client Then
			client.stream = CreateSocketStream(client.socket)
			Return client
		Else
			Return Null
		End If
	End Function
	
	Method onPacketReceive(packetType:Byte, packetData:Byte[])
		Local str:String
		For Local i:Int = 0 Until packetData.length
			str :+ Chr(packetData[i])
		Next
		Print "Received packet type: " + packetType +  " with data: " + str
	End Method
	
	Method onRecieve()
		If SocketReadAvail(Self.socket) > 0
			Self.buffer.Append(Self._ReadBytes())
			Local size:Int = SizeOf(Self.buffer.bytes)
			
			If size > 3 Then
				Local packetSize:Short = Self.buffer.ReadShort(1)
				If size - 3 >= packetSize Then
					Local packetType:Byte = Self.buffer.ReadByte(0)
					Local packetData:Byte[] = Self.buffer.ReadBytes(3, packetSize)
					Self.buffer.Pop(size)
					Self.onPacketReceive(packetType, packetData)
				End If
			End If
		End If
	End Method
	
	Method _ReadBytes:Byte[]()
		Local bytes:Byte[]
		While SocketReadAvail(Self.socket)
			Local l:Int = SocketReadAvail(Self.socket)
			bytes = New Byte[l]
			For Local i:Int = 0 Until l			
				bytes[i] = ReadByte(Self.stream)
			Next
		Wend
		Return bytes
	End Method
End Type

Type TBytes
	Field bytes:Byte[]
	
	Method Write(bytes:Byte[])
		Self.bytes = bytes
	End Method
	
	Method Append(bytes:Byte[])
		Self.bytes = Self.bytes + bytes
	End Method
	
	Method Pop(length:Int)
		Self.bytes = Self.bytes[length..(SizeOf(Self.bytes)-length)]
	End Method
	
	Method Read:Byte[]()
		Return Self.bytes
	End Method
	
	Method ReadBytes:Byte[](offset:Int, length:Int) 
		Local temp:Byte[] = New Byte[length]
		For Local i:Int = offset Until offset+length
			temp[i-offset] =  Self.bytes[i]
		Next
		Return temp
	End Method
	
	Method ReadByte:Byte(offset:Int) 
		Return Self.bytes[offset]
	End Method
	
	Method ReadShort:Short(offset:Int)
		Local  str:String
		For Local i:Int = offset To offset+1
			str :+ Chr(bytes[i])
		Next	 
		Return Short(str)
	End Method
	
	Method ReadInt:Int(offset:Int) 
		Local  str:String
		For Local i:Int = offset To offset+3
			str :+ Chr(bytes[i])
		Next	 
		Return Int(str)
	End Method
	
	Method ReadLong:Long(offset:Int)
		Local  str:String
		For Local i:Int = offset To offset+3
			str :+ Chr(bytes[i])
		Next	 
 		Return Long(str)
	End Method
	
	Method ReadFloat:Float(offset:Int) 
		Local  str:String
		For Local i:Int = offset To offset+3
			str :+ Chr(bytes[i])
		Next	 
		Return Float(str)
	End Method
	
	Method ReadDouble:Double(offset:Int) 
		Local  str:String
		For Local i:Int = offset To offset+7
			str :+ Chr(bytes[i])
		Next	 
		Return Double(str)
	End Method
End Type

and then with strings:

SuperStrict

Graphics 640, 480

Local client:OClient = OClient.Create("127.0.0.1", 6121)

While Not KeyHit(KEY_ESCAPE)
	client.onRecieve()
Wend

Rem
Function ToString(bytes:Byte[])
	Local str:String
	Local char:String
	Local i:Int = 0

	While i< bytes.length
                char = Chr(bytes[i..i+1])
		str = str + char
		i = i + 1
	Wend
	Return str
End Function
EndRem

Type OClient
	Field host:String
	Field port:Int
	Field socket:TSocket
	Field stream:TSocketstream
	Field buffer:TBytes
	
	Function Create:OClient(host:String, port:Int)
		Local client:OClient = New OClient
		client.host = host
		client.port = port
		client.socket = CreateTCPSocket()
		ConnectSocket(client.socket, HostIp(host), port)
		
		If client Then
			client.stream = CreateSocketStream(client.socket)
			Return client
		Else
			Return Null
		End If
	End Function
	
	Method onPacketReceive(packetType:String, packetData:String)
		Print "Received packet type: " + packetType +  " with data: " + packetData[..SizeOf(packetData)]
	End Method
	
	Method onRecieve()
		If SocketReadAvail(Self.socket) > 0
			Self.buffer.Append(Self._ReadBytes())
			Local size:Int = SizeOf(Self.buffer.bytes)
			
			If size > 3 Then
				Local packetSize:Int = Self.buffer.ReadShort(1)
				If size - 3 >= packetSize Then
					Local packetType:String = Chr(Self.buffer.ReadByte(0))
					Local packetData:String = Self.buffer.ReadBytes(3, packetSize)
					Self.buffer.Pop(size)
					Self.onPacketReceive(packetType, packetData)
				End If
			End If
		End If
	End Method
	
	Method _ReadBytes:String()
		Local bytes:String
		While SocketReadAvail(Self.socket) > 0
			bytes = bytes + Chr(ReadByte(Self.stream))
		Wend
		Return bytes
	End Method
End Type

Type TBytes
	Field bytes:String
	
	Method Write(bytes:Byte)
		Self.bytes = bytes
	End Method
	
	Method Append(bytes:String)
		Self.bytes = Self.bytes + bytes
	End Method
	
	Method Pop(length:Int)
		Self.bytes = Self.bytes[length..(SizeOf(Self.bytes)-length)]
	End Method
	
	Method Read:String()
		Return Self.bytes
	End Method
	
	Method ReadBytes:String(offset:Int, length:Int) 
		
		Return Self.bytes[offset..(offset+length)]
	End Method
	
	Method ReadByte:Byte(offset:Int) 
		Return Self.bytes[offset]
	End Method
	
	Method ReadShort:Short(offset:Int) 
		Return Short(Self.bytes[offset..(offset+2)])
	End Method
	
	Method ReadInt:Int(offset:Int) 
		Return Int(Self.bytes[offset..(offset+4)])
	End Method
	
	Method ReadLong:Long(offset:Int) 
		Return Long(Self.bytes[offset..(offset+4)])
	End Method
	
	Method ReadFloat:Float(offset:Int) 
		Return Float(Self.bytes[offset..(offset+4)])
	End Method
	
	Method ReadDouble:Double(offset:Int) 
		Return Double(Self.bytes[offset..(offset+8)])
	End Method
End Type

As you can see the one with string is a lot simpler but only because it is done by the compiler, unlike the one with bytes that everything has to be coded.

note that I have not tested so there might be some logic errors.

[edited]

look at this code:
http://www.blitzmax.com/Community/posts.php?topic=81750#921145
In principle it does the same thing but with tbanks and is alot simpler