The command-spacebar shortcut key isn working for play preview?
If you are using X.4, the system has added a global shortcut key that pre-empts MetaSynth’s. Go to the the Keyboard & Mouse System Preferences. Click on the Keyboard Shortcuts tab. Show Search Field probably has the shortcut set to command-spacebar. Turn this shortcut off or change it to something else for the shortcut to work in MetaSynth.