Make sure you follow the instructions...

After installing and updating and patching.. Go back to the wow folder and under Data, find the realmlist.wtf and edit it back to the private server one. After updating blizzard changes it back to the retail version so you have to double check it.

And also, the retail wow just updated to 3.2 patch.. Make sure you didn't update to that one. If you did, you have to downgrade back to 3.1 or just find another server that supports 3.2 version.