A serial number can be registered only once. When you register for the first time you select a username and password that will be associated with that serial number for ever. The username can not be changed (except under extra ordinary circumstances and only by contacting the helpdesk) but the password can by clicking on UserCP above or going directly to Planet Auran.
If you have forgotten your original username/password you will have to contact the helpdesk by using the link at the top of the forum page and give full details of where and when you purchased the product and provide the serial number for verification plus any other relevant info that the helpdesk requests. It is at the helpdesk technicians discression whether the request will be accepted or rejected.
If the username you are using for this forum is the correct first id used when registering then use that when configuring the game for DLS access. To verify that the serial number is indeed registered to the username you are currently using, click on Serial Number at the top of the forum and you see all registered serials to this account.
In short, once the game is loaded do not press join planet auran - simply enter the username, password in the appropriate field by starting the game from the desktop icon then pressing Options then click on the Planet Auran tab.
Hope this helps
Brian