Fear not, as your I-PAC is most likely wired correctly. By default, P2 B5, B6, B7, and B8 are mapped to the same buttons as P3 Up, Down, Left, and Right (in that order). Usually, this isn't a problem, as there are very few, if any, 3 or 4 player games that use more than 4 buttons (I think).
You can use the I-PAC software from the Ultimarc website to change the codes.
