NAMD Wiki: KwikiUserName
The username is saved in a cookie, so it should stay around after you are done with your session. If you happen to be using a public machine, you may wish to clear the username when you are done.
By default Kwiki requires that you create a page about yourself prior to setting your username. So if your name is Eddy Merckz, you should create a page called EddyMerckz, and describe yourself a bit. Then you can go to Preferences, and set your username.