This wouldn't have anything to do with your question here would it? Keep in mind about what I said in that thread about not doing this on systems that don't belong to you.
There are tools on BackTrack that can be used to capture traffic, but you appear to have a misconception about the ways in which this could work, and you will need some networking knowledge to understand this properly. Some packet capturing tools are Wireshark, tcpdump and Ettercap, however the way these work is to capture traffic that is seen by the system on which the tools are run. Ettercap has an ability to influence the operation of the network it runs on so that it can see more traffic than it would when the network was operating normally, but there are limitations to this technique. You will need to do further reading on your own to work out how to use these tools.