Ok...this seems to work for me...i use OMS and a USB Midiman interface on a G4. I have my OMS settings for Output and Input in set to the XP-50, and SYNC is getting sent from the XP-50 as well.
Make sure your XP-50 is allowing receive messages...check in your SYSTEM -MIDI menus on the XP-50.
If you want to set patches, in the Instrument List in Finale, choose Bank Select 32, Program Change. For example, if you wanted PR-C:38 , type 2 in the Bank, and 38 in the Program.
Bank 3 would be all GM patches on the XP-50...Bank 1 would be Bank B...etc...
Well, at least it works here...i dunno if that is BY the BOOK, but it works for me.