It seems RMX only permits a Folder structure up to 2 layers deep within the SAGE/User Libraries folder itself. This does not include the final layer with the data.xml file and RMX's own folders with "Audio" in them.
This would make sense, as the plugin only has that many layers in its own display.
If I try to select a folder within the plugin window which is deeper within the hierarchy, it won't work- the mouse just "bounces" off it. Reorganizing the folders within the maximum of two layers allows the files to show up (after closing & re-opening RMX).
What's really nice is that the SAGE converter converts everything (all REX files) within a drag-+dropped folder, saving a lot of multiple-selecting with the mouse.
It's actually really simple- and essential really if you have a large quantity of converted files- to create your own library structure as shown by Eric in the QuickTime tutorial #17.
Maybe this will shed a little light where needed for those just getting into this.
(edit) -just one thing...it seems a pity that RMX has to create its own copies of the .rx files; they're not that big but it could be a drawback on a smaller (eg.laptop-based) system
and finally...what a pleasure it is to browse using the RMX interface after fiddling around in the Kompakt and Kontakt browsers.