Yes, it should be adjustable.
The center screw is for locking it down. The two outer holes should have small screws with allen (hex) heads, that are used to raise the nut, perhaps differently at each side.
The procedure is that you loosen the center locking screw, then adjust the outer screws to your liking, then tighten (gently) the center screw when done.
The outer holes should already be threaded; the trick will be finding proper screws. But look carefully again - maybe you just didn't see them?
If there really are none, then your nut must already be down as far as possible, and to go lower you would have to remove some material from the back of the nut.
-Bob