In the absence of a MS decoding plug-in, and in case you didn't already know, the formula for deriving AB stereo from MidSide is A = M+S and B = M-S.
So put the M signal on one track feeding two outputs (A & B) and the S signal on two tracks (one panned to A the other phase inverted and panned to B). Level of S signal will affect the 'width' of the stereo image.
Good luck finding the phase invert plug in for FCP though! (I haven't looked very hard - hopefully there's one in the standard plug-ins).
HTH
Trevor