Assuming you've checked the Full Screen options I mentioned above, then the other thing I'd try would be to set the playback plugin to one of the defaults. It could be that it's working, but the display plugin is crashing so it looks like nothing's changing.
Try this:
1. Start some music playing back.
2. Switch to Playing Now manually by choosing it from the tree.
3. Go to this menu: Player --> Display Options --> Set Display --> Visualizations --> Cycle All. (Just to set it to something simple.)
4. Stop Playback. Verify that Options --> General --> Behavior --> Jump on Play is set to Display View (always) and that the Display Options mentioned in my above post are set normally (Nearest Monitor and Desktop Resolution).
5. Start a MP3 or similar file playing back. Don't use a file type not handled internally or by a DirectShow plugin, such as a M4A or M4P file.
That should give you Display View without a doubt. Now, try setting the "Set Display" option back to what you were using before. If this causes it to "stop working" then I think you have your culprit.
If it happens to be G-Force, there are some issues with G-Force and MC12, particularly with older builds of G-Force. If so, post back and I can help with them (but update to the newest build of G-Force first).