Unfortunately this interface has been suffering from some neglect.
At one point the JRDF interface broke ACID requirements on the database. Because of this, it was deprecated. However JRDF is a popular interface for Mulgara, so it has been repaired.
The old web site has some documentation on how to use JRDF, however this is a little out of date. People have reported success in using it, though I don't know what is needed to get it running now.
There are developers on the mailing list who use JRDF, so please ask if you have questions, and we will prod them until they respond.