how are u recording the midi? do u have a keyboard connected to a usb midi interface?
make sure that all your midi connections are correct and that OMS shows all your current set up. Is Quicktime on or off in your OMS set up? Double click on the Quicktime logo to check.
Can you hear anything when you are playing the midi through Quicktime?
You may need to reinstall OMS if nothing works. the most current version is 2.38 but some people have less problems with 2.37.