I've been playing around a bunch with RoboRealm, but I am using a different configuration than you, so unfortunately, I can't be of much help.
( mine will be: PC running RoboRealm --> XBee ...... XBee --> PICAXE Microcontroller--> L293D Motor Driver )
Steven at RoboRealm is pretty good about helping folks out... If you don't get any replies here or at your posting on the RoboRealm forum, I would try sending an e-mail.
Have you tried breaking down the problem into small parts? First confirm that your communications work, then confirm your motor controller. Do you have a spare microcontroller? One idea is to first create a roborealm program that will send a serial value to a microcontroller, then echo back the value to the PC. That will help ensure that you communications are good. Then do a test with just a microcontroller and the Sabertooth and confirm that your motor controller works.