If you're looking to play on a Cataclysm server, you will need a Cataclysm Client. It's typically not hard to get, but the clients are very different (some are working while others aren't). Today, I'll show you how to get it working.
Downloading the WoW Cataclysm Client
Now you will need to download the client to launch the game. The client depends on the expansion. If you have chosen a Cataclysm server, you will need to Download WoW Cataclysm since it's a 4.3.4 version. After extracting, make sure to update your realmlist.wtf file to the server you want to play on.
Conclusion
Now you're good to go, and ready to play on the WoW Server you've been wanting to play!
Downloading the WoW Cataclysm Client
Now you will need to download the client to launch the game. The client depends on the expansion. If you have chosen a Cataclysm server, you will need to Download WoW Cataclysm since it's a 4.3.4 version. After extracting, make sure to update your realmlist.wtf file to the server you want to play on.
Conclusion
Now you're good to go, and ready to play on the WoW Server you've been wanting to play!