how to add a walk around controler,and does it have to be atlas brand?
According to information on the Atlas web site, the Commander will support up to five hand-held controllers -- and yes, only Atlas controllers will work with it.
http://www.atlasrr.com/handcommand.htm
As I recall, the Atlas Commander is made by Lenz, and Lenz throttles may be compatible. If you look at the Atlas throttle pictured in the link in the previous post, you'll notice the remarkable resemblance to the Lenz LH90 throttle.
This is Lenz's "XPressNet" page: http://www.lenzusa.com/xpressnet/index.htm
You'll note that it contains Lenz, Atlas and CVP products. It claims they are all inter-operable. I've got a Lenz system myself, but I only use Lenz throttles.
If this is really the case, and you can probably verify it by contacting Lenz, I would strongly recommend the LH100 throttle. The throttle's "stack" holds 32 addresses. Mine has been in service for 6 years now, and it still works perfectly. I also have an LH90, similar to the Atlas walkaround HandCommand unit, but it's awkward to use for anything more than running a single locomotive.
The DIN plugs for the Lenz throttle bus are commercially available at electronics places for a couple of dollars. Instead of the overpriced panels, I bought a bunch of these and mounted them on my fascia, connecting them with 4-conductor phone wire. I've put jacks about every 5 feet around my layout, so I can plug in my throttles anywhere.
The CVP unit is a wireless throttle, if you're interested in that option.
It takes an iron man to play with a toy iron horse.
Lenz throttles(LH90/LH100) will also work with the Commander. You would need to use a Lenz LA152 adapter to convert from the 5 pin Din connnector on the throttle to the RJ12 jack on the Commander.