I have NO idea if this is even CLOSE, but I just got a Performance Pad Pro, and I think it's a similar engine... On it, there is a Page UP, Page Down, for each pad. Use that to change the pages in the "Drum Set" screen, and one of the pages (Page 3 on the Perf Pad Pro), has the instrument assigned to the pad.
Now, if I could just figure out how to save a kit... it won't let me!!!