Well it depends what you installed. Did you install on mdk still? For the linux-wlan-ng, you also have to patch the source before you compile and install it, like it says in the airsnort readme. Or else it is lacking the ability to be able to listen in promiscuous mode. Having said that, once everything is installed properly, there is a shell script with the commands to put it into promiscuous mode in the airsnort package. Here is the command from the script.

--- Begin /airsnort-0.1.0/scripts/dopromisc.sh ---

wlanctl-ng wlan0 lnxreq_wlansniff channel=6 enable=true

--- End /airsnort-0.1.0/scripts/dopromisc.sh ---

Hope this helps

-- ZeroBreak