You didn't say what type of computer(model) you're using. The BackTrack version that you are using is an older one and you probably should upgrade, as BT is on 5r3 now. With regards to your wireless card, from what little i could find online it appears that your card does support monitor mode. It could be an issue of installing the drivers for your broadcomm 4313(do some research on your own). Make sure that in the Applications/ Wicd Network Manager/Preferences/Wireless interface that you type in: wlan0 in the space provided so that backtrack will look for a wireless 802.11 card. Good luck and don't give up.