Combining stiff mechanical items with squishy organic things is a challenge for Poser's rigging, to say the least. At a glance, I'd say that the pivot point of the leg brace isn't at the same point as M3's leg joint bend center. If you go into the joint editor for M3's shin bend (not the brace) you'll see where the center point lies. You'll probably need to remodel the brace so that its joint is at the same place.

Once you have that right, you may still have trouble getting the joint parameters right - but better riggers than I can advise on that.

Oh, and if you meant to post this in the Poser forum - sorry, you missed. This is the 3D modelling forum. ;)

Oh (2) - you can add an image to your post by using the "Attach a File:" box below the Reply section.