This may look complicated, but is super easy. If you don't feel comfortable using this method, you can use this automated tool (yes, it works on 4.5.141). It won't be as satisfying, or even feel good. You won't learn anything, but you will be rooted. Download tool here (link in OP is broken).
1) Read the xda Atrix Beginners Guide (then read it again)! (Link)
2) Download and install 7zip. (Link)
3) If they are not already installed on your PC, download and install the Atrix drivers for Windows. (Link)
4) Download the attached files (sorry, had to break them in to two files due to forum size constraints). Create a new folder on your hard drive named "Android" (c:\Android). Extract both attached zip files to this folder (all the files need to be in "Android", not in a sub folder).
5) On your phone go to Settings, Applications, Development, then tick USB Debugging.
Do not type quotes.
After each command hit enter.
Be mindful of spaces.
You can simply copy each command and paste in to the command prompt.
If you encounter any errors after entering a command, just ignore and keep going (assuming you entered the command properly).
1) Attach your phone to the PC via USB. In the USB settings make sure USB Mass Storage is selected.
2) Reboot phone while holding volume down until you see "fastboot".
3) Press volume up to initiate fastboot.
4) On your PC click the Start menu, then in the search box type "cmd"
5) In the command prompt type "cd c:/android"
6) "fastboot.exe flash preinstall preinstall.img"
7) "fastboot.exe reboot"
8) Wait for phone to reboot.
9) "adb shell"
10) "/preinstall/dosu". If you get an error (I did), type "/preinstall/su"
11) "/bin/mount -o remount,rw /system"
12) cp /preinstall/su /system/bin/"
13) "chmod 6755 /system/bin/su"
14) "PATH=/system/bin:$PATH pm install /preinstall/Superuser.apk"
15) Reboot phone.
16) On your phone go to your apps and make sure the Superuser icon is there. Open the app, go to settings, then update the SU binary.
17) Open a terminal emulator, then type "su" and hit enter. You should see "#" if you are rooted. Or, download Root Checker from the Play Store.
My phone rebooted itself after step 11, so I had to re-open the adb shell.
I copied and pasted the text from the command prompt for reference (attached).
If I missed anything or have any errors, let me know and I will update the post.
Note: If you experience problems with updating superuser after rooting, try going in to Manage Applications and clear Superuser data. If that doesn't work go to the Play Store and Install Superuser Update Fixer.
My Atrix AFH Folder