Install the plugin. You only have to install the plugin one time. After you do it once it will always be there. Installing a plugin is a 2 step operation. The first step is to download the plugin from the ichat site to your computer.
Second step is to install the plugin.
Once you do that the install will begin and it is pretty much automatic. After the install is complete you can delete the file you downloaded if you want to save disk space on your computer. The username can be whatever you want it to be - this is the name everybody else is going to see whenever you type something and chat. Some people just pick their name like 'Drbradley' and others pick a name that is a personna like 'cheery'. The registration screen also lets you optionally add your email, URL, phone number, address, and comment.