Looks like it is Linux only for starters.

Down in his forum
I found a message from Scott (Proj Admin) back to Saenger
I'm still trying to get the kinks worked out with this new application. Actually, nobody was supposed to see this stage of the project, but account creation was opened temporarily by accident for about a day.

It will be a little while until I've got everything working, so please be patient. I would recommend, though, that you don't attempt to perform any Cosmology@Home work until I make the announcement that we're good to go.
36 people managed to get in in the accidental 24 hour window. Heh!
Some of them have already got points. Boo!