Click the Chat Button. It is usually there in all designs.
It should look like this.
And do NOT forget to vote!
For people wanting to use another client like mIRC
If you want to register your own Nickname you need to use the Nickserv Command.
Help about that can be found here
Guide for connecting to IRC can be found here:
[Guide] IRC for Dummys. Join #D3scene now!
You only need to change the servers to NexusIRC.
See you there soon.