I had been having trouble getting everything to run smoothly in the new iMac lab we are setting up. But, this fixed my issue with launching DirectRT from MediaLab. (Note: I only had to fix the registry entry for directrt.exe, executeDRT.exe was fine.)

And, just FYI for anyone else using Macs, we also found we had to lower the screen resolution on the iMacs in order for MediaLab to run. I'm not sure if that's already a known issue or not. (We are running v2010 of both ML & DRT.)