If you can't see the "change nick" box in the bottom right corner, you can change your nick by typing /nick yournick and pressing enter.
If you've got an error like that: Unable to connect : java.security.AccessControlException : access denied (java.net.SocketPermission irc.worldnet.net resolve) and that you're sure to have said yes to the certificate when it popped up, try with another browser. If you've not, then clear your browser cache and/or refresh the page, and answer yes.