Do you have DDUtil running to direct the CAT commands? (Some folk mention using other programs like OmniRig, but I've not done so personally.) It appears that RDXcluster might look to a middle program to transport the CAT commands. Documentation is pretty sparse....
Here is a RDXclunster link
http://www.mdxc.org/swl/art-rdxcluster.phpand
DDUtil at
http://k5fr.com/ddutilwiki/index.php?title=Main_Page73
Steve
K9ZW
Blog: With Varying Frequency - Amateur Radio Ponderings
http://k9zw.wordpress.com