there is an issue with MB Looks 3 and FCPX, which probably affects all their suite. I have just finished an exchange with their support which confirms it.
The issue is that the preview that you use to choose and tweak the particular look you want is ALWAYS taken from the original media/clip. It will not show any effects you have applied to the clip in the timeline, it does not even show cropping, scaling etc. You can still use the software, but you have to guess what it is going to look like then exit the Magic Bullet interface to look at the result from the timeline.
They say they do not think they can fix this - I have said that they really should let potential FCPX customers know - it is probably a deal-breaker, especially for such an expensive plug-in.