Float may be what you need, but there's also seat height, cleat position, seat position and crankarm length (probably in that order). Have you got to some of the threads that discuss those? Did your LBS set your seat height and install the cleats onto your shoes, or did you do it yourself? What kind of pedals did you get?

+1 on posting in the Health thread and/or PMing one of the PTs if you want more specifics. Take care of that knee! Hope it feels better soon!