Yea, I guess mame couldn't find the file bulbolb in the current dir.
You could also do that (if mame supports) :
ExecFile("c:\mame\mame.exe C:\mame\bulbbolb.nes")
You could also do that (if mame supports) :
ExecFile("c:\mame\mame.exe C:\mame\bulbbolb.nes")