After that place it on your SD card. Then boot into recovery. Flash zip from SD card, choose the superuser zip, and you should be rooted.
Based on the info in this topic, I unlocked my Atrix, but still cant get root to set.
I followed your suggestion accordingly, and it seemed to work fine. However, root didnt take.
I'm able to mount the /system dir in rw mode via adb shell, and tool around using Root Explorer on the Atrix itself.
When I list the /system/bin dir in the shell, I cannot see su in there, however Root Explorer on the Atrix can see an su there. The su file is always zero byte in that dir.
For giggles, I renamed it "su.old" for some experimentation.... and within 30 seconds, it was magically renamed back to "su" all by itself.
Theres clearly some service/daemon/app with a choke hold on /system, which is guarding its fake "su" in bin, and also lying about free space on the mount.
I'm really quite stunned ATT and/or Motorola would waste so much time on such a petty thing, yet cant be bothered to keep their updates promise. Shameful behavior...