If Backtrack is based on Slackware (as this
says), then you'll want to look in /etc/rc.d to get the DHCP client to run. Specifically (and this is for Slackware), the file you want is rc.inet1.conf, where you can choose to use DHCP for your interface(s). For example, my eth0 section of rc.inet1.conf looks like:
# Config information for eth0:
This won't actually run the DHCP client, that is done by rc.inet1, which first checks the config file to see if USE_DHCP is set to yes and if it is, the client is run.