I had to split the outer plastic on mine in order to move the shaft back and forth to hook it up, but I can get the grease gun on without moving the hub back and forth. Even with the shaft split, the hub still doesnt move back, just rotates so I cant help with how to move it back. I think it must slip on from the open end and snap in to place somehow as it is firmly in place.
If you cant get the grease gun on, I would cut a notch or a hole in it as suggested so I could line it up. THe grease zerk should be one with a 90 or 45 turn on it so it faces out. On one of my BH, there is only one point in 360 degree of rotation that the shaft has to be turned to to access the zerk, anywhere else and the knuckle joint is too tight when it is hooked to the tractor but no problem with the safety cover.