I'm scratching my head over this, but it says I need to be logged in, in order to write messages/comments, although I can access the chat page and read it!
Any clues as to why this might be so?
Chat is public (well, the parts you see anyway), so anyone can read messages.
Aside from that, you can be logged in to physics.SE but not chat.SE. You need to log in from here. If that doesn't work, you can see if your browser is incompatible(try the GlobalAuth test link on the page).
Otherwise, you can first log in normally here, and then open chat.