Hi all. I just got tethering working on my Atrix yesterday. You need the latest version of Wireless Tether (2.0.7 as of this writing). Go to:
Downloads - android-wifi-tether - Project Hosting on Google Code
and download "wireless_tether_2_0_7.apk". Install that badboy onto your Atrix and you should be good to go. Of course, as it's already been mentioned, you must have root access into your phone.
I first tried with 2.0.6, but couldn't get it to work, and then I went to download one of the 'experimental' releases, only to find that version 2.0.7 had just been uploaded less than one minute earlier!
So I installed that one and BOOM! I'm tethering like a pro!
And hey, may I just say that the developers of Wireless Tether work really hard to bring us such a great thing. Their labors save us a lot of money. If it works for you and you decide to keep using it, please do consider throwing them a bone. Just think of all the money that you would otherwise be spending to tether. Just sayin'... I don't normally pay for software, but these guys deserve our support.