You can only ever capture traffic that is actually passing through the box you are listening on. Promiscuous mode only means that if traffic happens to arrive at your box but is not addressed to it, it would read that traffic. You are only going to get into that situation if you are using a network hub however. On a switched network, only traffic actually intended for your machine ever gets to your machine.
You would need to use a switch that has a SPAN port in order to monitor all the traffic going through that switch, or run your monitoring program on a system that all your network traffic passes through (a machine on the border of your network, for example).