Just replace "forumname" with the handle you use on the forums (or mainpage) for ease of others knowing who you are.
To register your nick, type in: /ns register email password and replace "email" with your email address, and "password" with what you want as your password. This only needs to be done once; every time thereafter, you only need to have it identify you. That is done by entering: /msg nickserv identify password entering in your password in place of "password."
Identifying can also then be done when you sign in, by checking the "Identify to NickServ" box, and entering in the password in the new text entry box:
Once signed in there will also be a couple tabs up at the top, and more will open when you private message someone. You change tabs simply by clicking on them.
Over on the right hand side there is also a list of those who are in the IRC room. You can start a private message to someone by clicking their name (left click, not right click) and selecting "query" from the list.