Sometimes the part of the MMG that fits onto the receiver can shrink a little making it tight to fit onto the receiver.  Either pry it open with a screw driver and slip it on or follow the images below.  You can insert the receiver into the widest part of the opening which is near the bottom of the gap.  Then push it on.  It will force the rest of the gap to widen as you push it on. 

This is a common occurrence with AR grips, not just the MMG.  Not all are like this, but a few do sneak by and they will fit, though you need to follow these steps to make them fit.  Please contact me if you simply can not get it on.  Thanks.