Just a guess here, but it could be a permission problem as crontrab may not execute under your userid (i don't know). Also, the execute bit may need to be set (again, just a guess; don't know).
Also consider the possibility that your script is indeed running, but that it's getting an error under cron that it doesn't get when you run it. From your error message, Finder got an error: Application isn't running, it seems to me this might indeed be the case (as the error appears to be coming from the Finder; does your script call the Finder to do something?). You might ask the script to do something simple like beep to verify whether or not it's actually being called. Again, the reason might be that cron could be executing under a different user id.
Good luck (and please post the answer when you find one).