In no order as they come out...
Title attributes or tooltips for all buttons. I don't know what they do.
Fade the modal in/out faster, make hitting ESC or clicking the faded are close the modal (the X is rather small)
More importantly, the whole way you manage going to URLs (and "freezing") is very unintuitive and backward. For starters, the arrow to "type in a URL ... [as far as I intially read]" points to an unfamiliar icon. I go there and see "freeze" and "unfreeze" which makes no sense to me. Freeze what? It was not clear that the main window was meant to show a URL rather than being an introduction to the service.
Furthermore, I'd never expect that typing a URL in chat would control the window on the right. It's a chat box. I type in it, and what I type goes into the chat log.
I greatly suggest the following:
Put an address bar above the window, or in the window, and next to it a "lock" or "freeze" button which says "prevents this URL from changing". Furthermore, it'll pay dividends having this bar up there when you add more options.. like maybe "new tab" and things like that.
Initially have the URL be the actual URL of your "welcome" page... (eg: http://chatterbloc.com/welcome_to_new_room.php) It will immediately tell me two things: "oh I can change the URL" and "I'm currently at the welcome page, lemme see what it has to say."
Explain features and stuff at this welcome page.. like that I can type "goto <url>" in chat to navigate . And that I can prevent navigation by freezing by locking the URL.
Make back and forward buttons.
Finally, let me name the room and give me a short URL.
All in all, a nice little service akin to Gabbly, Twiddla, etc. I like the iciness of it.
Hope this helps.