There was no visible contact address outside of the forum to sort this out. I even asked on Joomla! forums but nobody could (or was prepared to) help get me into the forums. I gave up.
Re-visiting this today (Dec 2009), I see that it's now up to version 2.1 so I thought that I might give it a try and see if the registration issues have been resolved. Sadly, it's even worse with the web site totally inaccessible and reporting:
"509 Bandwidth Limit Exceeded
The server is temporarily unable to service your request due to the site owner reaching his/her bandwidth limit. Please try again later."
How on earth does RSGallery2 get a POPULAR rating on Joomla! when it's so inaccessible? I think Joomla.org should exercise a bit of quality control here and only permit links to extensions which are a) available with valid links and working web sites, b) have demos available, and c) have some kind of basic support that is actually usable.
Sorry to be so negative but promoting an inactive extension just wastes my time and those of others too.
We recently transitioned the project to a new development team and they had some issues with their website hosting provider.
So yes, RSGallery2 is alive! Feel free to visit the forums and ask any questions you might have: http://rsgallery2.net/