There is a very helpful document at the Roland site which describes how to use an external sequencer with the XP-50. I have an XP-60, but the information was still sufficiently similar that I could figure out what to do. The URL is:
http://www.rolandus.com/SUPPORT/DOCS/SUPNOTES/XPEXT.HTM
Hope this helps. Reply to this post or e-mail me if you have additional questions.