It's broken.  If you can manage to somehow get the debugger to start
on fixed port 1098, it should work.  Let me know if you do.  If you
look at the Trac tickets for the RDT project, there are several
debugger component tickets all saying the same thing.  All the
examples that show RDT working with variables shown all have the
debugger port set to 1098 in screenshots.

As I noticed on the mail list for RDT, the main developer for RDT will
join a commercial fork  in order to get greater financial support .

RDT is also in competition by the funded "Eclipse DLTK | Dynamic
Languages Toolkit" http://www.eclipsedltk.org/   DLTK for Eclipse has
some working ruby support in a recent developer snapshot, but not in a
stable release as yet.

Netbeans 6.0 is in the same boat.