Code is executed from TOP to BOTTOM. Your check if the answer is right comes BEFORE the user has typed it in. SO, guess will always equal 0 when the check occurs.
Move the two lines:
Locate 660, 320
guess = Input("")
before the line:
If number1+number2=guess Then
Move the two lines:
Locate 660, 320
guess = Input("")
before the line:
If number1+number2=guess Then