1. Be sure your WiFi card is compatible with BT4and/ or there are supported drivers. Of course many would recommend getting an Atheros card.
2. Check out Up and Running Backtrack 4 - Offensive Security and the 'Setting Up Wireless' chapter
(enabling the card- /etc/init.d/wicd start then going to Wicd manager...etc...)
I would heavily advise browsing the forums and reading the howto sections, etc...