Can't turn adj knob on Avid BB 5's
 

My rear brake has almost no stopping power. I tried turning the red adjusting wheel that moves the pads in and out and the wheel won't turn. Pretty sure I need new pads.
Any info on how to work on these brakes?

The best source for specific service info is usually the maker's site.

Or you can, search for tutotials on the same job ie. adjust bicycle disc brakes and fine one for the same general type of brake, such as mechanical vs. hydraulic.

Odds are your adjuster is frozen either because it was threaded to one end hard against the stop, or corrosion on the threads. Some force may be needed to free it, but first you have to determine which way to turn.

that is the stationary pad adjustment, just the stationary pad ..
in addition to the red knob, Alternately, use a Torx driver on the bolt head in the center ..


The moving pad is adjusted with the barrel adjuster on the cable housing ,

or just the cable fixing bolt itself.. loosen, pull a little more cable through , and re tighten the bolt ..


If you need new pads , back out the adjustments first .. It might be easier to take the caliper off the frame
so you can clean the whole thing up well, then put in the new pads ..