Why can I not adjust the cutting depth on this router?
Ok, revised my answer

I found the instructions here and the information is on page two under "Set-up: Fixed and D-Handle Base - ADJUSTING THE DEPTH OF CUT (FIG. 1, 3)"

Occurring to this there is no lock of the black adjustment ring so it should turn somewhat freely, though it might have some resistance so not to move when you don't want it it.

It's very possible that your adjuster (piece B) is stuck. As it is, it is 'bottomed out' at max depth setting and it might have been tightened in this direction too far causing it to be stuck. If you feel save that you won't break it, turn the adjuster in the direction of the grove going away from the power head, this should be obvious. Once unstuck it should turn freely 

If this doesn't work, or you don't feel comfortable trying the above, then return it as it's not working as intended.