Work your way up the OSI stack to troubleshoot. First confirm the wireless card has associated with your AP. Then confirm you have an IP address assigned. Then confirm you can ping that IP address, then ping your default gateways address, then ping a machine on the Internet by IP address. Check DNS resolution works. Confirm no firewall is filtering your traffic.
Assuming you have a simple network that is connected to the Internet properly, the most likely cause of the problem is that you haven't setup the wireless interface in Backtrack properly. Searching the forum here will help you find information on how to do this.