Select VIEW...COMPILE AND PROGRAM OPTIONS, the the asembler tab. Manually point it to MPASMWIN.EXE. If you are still having problems, you need to manually edit the registry and delete

HKEY_CURRENT_USER\Software\MecaniqueUK\MicroCodeSt udioPlus\Assembler

OR

HKEY_CURRENT_USER\Software\MecaniqueUK\MicroCodeSt udio\Assembler

if you are using the free version. After that, all should work well from within MCS