Paste in the appropriate "set realmlist" entry from SnorkelPop's post above, on a blank line, with no spaces at the beginning or end of the line (extra blank lines above/below aren't a problem, though):
For US Clients — <World of Warcraft>/realmlist.wtf:
set realmlist us.logon.worldofwarcraft.com
For EU Clients — <World of Warcraft>/realmlist.wtf:
set realmlist eu.logon.worldofwarcraft.com