Creating an executable jar file

Okay, I'm using JCreator for this. I've tried to create a jar file using JCreator, and it didn't cough up any errors. The manifest file is correct along with everything else, but When I execute it the "Java Virtual Machine Launcher" gives the following error: "Could not find Main Class. Program will exit."

Does anyone else have this problem? Jar files are pretty much portable, so I figured this was a good forum :).