Hi there, we love the Florida Mall, and you can get a Lynx Bus from the TRansport center at MK, with only one change. I believe you take the number 50 and change to the number 4 (not sure where - but you could find out on the bus) and this stops opposite the FM I believe. (I found this recently on the lynx website, so haven't had chance to try it yet - sorry).
The Florida Mall is really big and we try and take a trip there whenever we go!