Can't boot to Clockwork Recovery on Atrix (several methods tried)
Rooting the Atrix 4G does only that - gives you elevated privileges on stock OS.  In order to be able to install a different recovery image and custom ROMs, the phone's bootloader must be unlocked first.

This is a fairly complicated process that carries the usual warnings and disclaimers of modifying phone software.  It will most definitely void your warranty with AT&amp;T (and possibly with Motorola as well,) so be very certain that your phone is either free of hardware defects, or out of warranty with your carrier before unlocking it.

See this thread on xda-developers forum for further details and installation instructions.