1. You need to have your realmlist.wtf file set correctly. Using the simpletext applications Notepad or TextEdit open up the realmlist.wtf file in your WoW folder and change it to say exactly this:
set realmlist login.nostalrius.org
or
set realmlist login2.nostalrius.org
2. If you have been online and now cannot log in or are logging in to incorrect realm, the simplest solution is to close WoW and navigate to the WoW folder and throw away your WTF folder. Reopen WoW and you will be prompted to select realm again. You can also attempt to hit "Esc" while logging in to try to toggle yourself back to the realm selection screen.
3. If you have altered your Config.wtf and do not wish to throw away the WTF folder, or if for some reason throwing it away does not re-prompt you, then open up Config.wtf with your simpletext program and edit the below lines to say exactly this:
SET realmList "login.nostalrius.org"
SET realmName "Nostalrius Begins"
refer to this page for further suggestions:
viewtopic.php?f=45&t=5359