Ok Nat, hold on a sec.....
"now I have hooked up the XP via a SBLive with ASIO drivers, into a demo of
Logic Platinum 5"

right ok......this sounds fine, presumably you have your Midi in/out connected correctly (midi in goes to midi out and vice versa)

"So far so good, after fiddling around with the XP, I can play the
GM instruments and record in Logic."

If you were paying the sounds from the XP50 it would make no difference whether they were GM sounds or not...it is just another soundbank afterall and no different frm the pop expansion board. It sounds to me like you are acually hearing sounds that are coming out of your soundcards onboard synth.
What you need to do is make sure that you have the MIDI on your XP set to Local 'off' and set the appropriate box in Logic Platinum to external MIDI ( I think on the left hand side of the display when you have selected a track rather than it saying PC or whatever double click it and set it to external MIDI.)

"But I do not know how to get the sounds from the XP into Logic"

THis is where I think you may be getting slightly confused. MIDI simply passes mssages between your synth and yor computer and tells each one what to do e.g 'play C5 for 4 seconds gradually getting louder'. It does not actually transmit the sounds. The sounds stay in your synth and the sequencer just tells the synth what to play...it is much like a hightech pianola, whereby you create the music for your synth to play. Many people seem to be disheartened that the sounds don't go into the sequencer but MIDI lets you have virtually coplete control over every parameter of your synth and is therefore incredably powerful. hope this helps

Regards

Dr. O