I have now spent many many days trying to get this working and my discoveries are that it ONLY works with ubuntu 9.10 (karmic) 64 bit edition.
What I tried
I tried a fresh installation of 9.04 jaunty (32 bit). Installed RAD 7.5 (worked happily). Upgraded to 9.10 - Rad now FAILED
Fresh install of 9.10 32 bit (karmic). Installed RAD. Rad installs without issue but cannot create a workspace - FAILED
Fresh install of 9.10 64 bit (karmic). Installed RAD - FAILED
Fresh install of 9.04 jaunty (64 bit). Installed RAD 7.5 (worked happily). Upgraded to 9.10
WORKS with a mouse click error (can be fixed!)
My conclusion is that RAD uses 32 bit libraries which are overwritten by the "standard" 9.10 upgrade process when using ubuntu 32 bit . However, when using the 64 bit version of ubuntu, the 32 bit libraries needed for RAD are not overwritten and so it continues to work.
I hope this helps someone - it took me days to find this out. I guess eventually some clever guy will find out how to get RAD 7.5 working on 32 bit karmic, but for now. I can only get it working on 64 bit.