You just installed Burning Crusade for wow and it wont let you become a blood elf why is that

After installing the Burning Crusade, your account also needs to be upgraded to make use of the new event. Go to the BattleNet website (see related links), log in with your details, and find your World of Warcraft subscription. Here you will find an option to "Add Game". Simply put in the authentication key from your TBC CD, and your account will work.