Eclipse RDT preferences page not error



When I try to install the Ruby RDT (v7) for Eclipse 3.1.1 by
downloading the file and upzipping it into the Eclipse directory, the
plugin installs in that I can see it listed as a plugin, but the
Preferences page for it produces an error:

Unable to create the selected Preference Page

Plug-in org.rubypeople.rdt.ui was unable to load class

I can see that this file is in a zip file, but Eclipse can’t see it.

Any ideas?