DECLARE INTEGER ShellExecute ;
IN SHELL32.DLL ;
INTEGER nWinHandle,;
STRING cOperation,;
STRING cFileName,;
STRING cParameters,;
STRING cDirectory,;
INTEGER nShowWindow
*Syntax:
*? ShellExecute(hwnd,lpOperation,lpFile,lpParameters,lpDirectory,nShow)

I am not quite sure what do with your parameters. Try:
? ShellExecute(0,"open","C:\Program Files\TWAINCommander\TWAINCom.exe",;
"y:\myapp\mydoc.tiff -multipagetiff",;
"y:\MYAPP",1)

clear dlls

>ShellExecute("TWAINCOM.EXE", "y:\MYAPP","y:\myapp\mydoc.tiff -multipagetiff")
>
>file shellexecute.prg does'nt exist
>
>I want to run the twaincom.exe program from visual foxpro.
>
>twaincom.exe is located at the following path:
>"C:\Program Files\TWAINCommander\TWAINCom.exe"
>
>Thank you very much.