Jim,
The answer is in the owner's manual (well, not the whole answer...)
The Bank select box in Cubase's inspector window is the sum of two possible bank select (controller) messages. The Korg X5 uses only Controller 0 (MSB) or Most Significant Bit, for the bank change. The number you enter in Cubase is the number you'll find in the manual (sorry, I can't remember the number... (!) multiplied by 128. The General MIDI bank is bank 0, so the number to enter to switch to this bank is 0*128, or 0. I seem to remember 72 or something as the other bank, so, 72*128=9216. So enter 9216 (or whatever the product is). You can also select a drum kit for any channel in the same way.
Steve Martin
Ward Music