Ok, so finally heres my guide to connect to a hidden servergroup
1. Make sure you have version 2.4.2
2. Open the realmlist file in your WoW-Folder and change it to:
set realmlist 12.129.236.109
set patchlist eu.version.worldofwarcraft.com
3. Start WoW and use the following Account Information:
Acc: Blizzard
PW: Blizzard
4. Log in and you should see four servers
edit: And YES the Account is Gm-Tagged
Have Fun