I'm running RIA Developer on Eclipse 3.4, with RadRails 0.9.3.6479.
There are folks (Tejus,Nolan) who are running on 3.3 though (I believe).
Are you trying to install RIA Developer atop Aptana's custom branding of Eclipse (Aptana Studio)? I haven't tested that at all, and we likely won't be supporting that (since it's clearly missing standard Java plugins).
All this is moot though, as the next version of RIA Developer will remove the ant dependency, and use our command line tools for creating/building/updating/deployment projects.
--Mark