There are more authentication modules supported, so if you have accounts there, you can use them to login to chat:
There is a W button in the top left corner of this chat. Click it to connect. Right from this button, there is server selection, so you can connect to any server that is online. All servers are connected in one network.
Chat is not using your username as nickname and it is for security purpose.
After first login to chat, random nickname is generated for you. Then, you have to manually set your nick by typing /nick newNick as message to any chat window.
Nicks are unique and once you have your nick, nobody else can have same nick as you have. It is always possible to change nick anytime, if the nick is not used by anyone else.
Login to the chat and type /list as a message. This will return all channels available on the chat network. Channels (rooms) are marked with hash sign, so anytime you will see #something o this chat, you will know it is a channel (room).
Then, just type /join #channelName to join specified channel (room).
Sure, type /help as message and all commands will be listed.