These robots have a proprietary board inside called an rFlex bus that communicates with the sensors and motors. I've been trying to communicate with the rFlex board on my ATRV-Jr for a while now with no luck so far.
I found that the Washington University robotics group has written some ROS software that communicates with rFlex based robots, but it didn't seem to work for me. But I haven't had time to troubleshoot deeply yet. It's not my main project.
http://wu-ros-pkg.sourceforge.net
Here is a manual for the iRobot/RWI software that runs on the onboard PC. It's terribly old not supported anymore, so I did not find it useful.
http://www.cse.buffalo.edu/~shapiro/C...
I have been able to drive mine around the lab using an old wired joystick and walking behind it. I can talk you through that if you'd like to do that. Let me know if you're interested.