Well, what you really need to do if you want to play this is get the WoW Client WITH Burning Crusade AND Patch 2.3. You must first create a server account here if you haven't done so already:
http://www.wowfusiononline.com/index.php?option=com_wrapper&Itemid=31Then you go to where you stored WoW (C:\Program Files\World of Warcraft)
There you will find a file called Realmlist.wtf. You need to erase everything there, or better yet, save a copy of it somewhere safe if you ever want to play retail again, and replace it with "set realmlist burningfusion.servegame.com" without the quotes. Then you logon with the information you gave the site, select your realm (BuringFusion is the only one up now, but another is coming), create your character, and play like you normally would retail WoW.
This server is high rate, meaning, it is easier to level than retail version. The exp rates are 40x higher than retail, if I remember correctly. I got up to lvl 60 in about 24 to 30 hours totaled.
EDIT- Borat, I don't know the exact numbers, but there over 1200 on at any given time. Horde to Alliance ratio is about 3 Alliance to 4 Horde.