The tutorial from Peter Gannon looks pretty good. Notice down toward the bottom he shows loading the sounds into the slots and shows the midi channel number that's already assigned to that slot. For example, slot number two is assigned midi channel 2, slot 3 -> channel 3 and so on. To load a bass, for example, you would click on slot two and choose a bass sound to load it into that slot. To see the midi channel assignment in biab, open 'preferences' (control and e) and look out toward the top right and click a button marked 'channels'. That will show you the midi channel assignment in biab, then you just match it in jabb. I hope this helps, but if it doesn't, post back your questions.
http://www.pgmusic.com/tutorial_garritan.htm