The locking block is indeed very, very tight. There are two methods to fit it in the frame. You can use a 7/16" metal rod, shank of a drill bit, or otherwise similar sized instrument like a Sharpie Marker to place horizontally between the two nubs that stick up off of the frame. Pushing it down in between this area will open the cavity slightly and allow the locking block to fit.
These are the newest directions that have been put into our instruction manual, but I have found that if you can get the front legs of the locking block rail system squeezed and fitted, you can then tap it into the frame with a soft hammer or mallet. Freezing the rail for an hour or two can also make both of these methods a little easier.