The visual editor does not work. See: http://bestbits.net/wiki/main/.
I had a thread about this on the wpmu forums, but the response there was "it must just be a theme or plugin conflict". So I contacted the guys who had been working on the theme and some other development for me, and they just came back and said it's not just a theme or plugin conflict.
They said that the problem is that the visual editor code tries to load itself multiple times all over the page. You can actually see this if you click "edit". There are half a dozen (empty) visual editors everywhere. And none of them work.
They weren't able to fix it and suggested I come back to the wiki developers.