I'm pretty sure the 21.1 is learning. But you can't learn to the device selection keys like DVD. So just pick some other unused button. You probably don't use the colored buttons. So I'd use red for off and green (or power) for on since that's easy to remember.
If you get an IR adapter for your PS3, you can use your Dish remote to control it as well. The Nyko blu-wave, for example, works a PS3 using standard sony DVD codes.
Also make sure your remote isn't in limited mode. Otherwise most of the buttons won't work. All the programming details are in the manual you got with your system or are downloadable from Dish's site.