We suggest using your World of Warcraft - The Burning Crusade CD/DVD's legally acquired.
Otherwise, you can buy the game at Blizzards website, and use the download link below to download The Burning Crusade version:
Click to download WoW 2.4.3 The Burning Crusade
Click if above doesn't work
You will find this file in your WoW folder.
Edit this file, using Notepad, so it reads (copy & paste below):
set realmlist logon.hellfire-tbc.com