I brought the original WoW game and the month expired. I then bought World of Warcraft BC and installed it and now on my battle.net account status is frozen. If has been four days

When you install the game, you make an account that gives you a certain free time to play the game. After that, you will have to use a Game Card or a Subscription to continue to be able to play the game.

Regardless of installing BC, this does not add days to your account, it simply gives you access to more content.

The best thing to do is either make a Subscription through your battle.net account using a credit card or direct debit (if available in your country), or try and buy a Game Card in a game store. The instructions for using the game card are on the card itself.

Generally the subscription is the cheaper option, but the game card gives you more libery to determine when you pay for your play.