I think airtun-ng is what you're looking for.
Don't forget to start the tun driver by typing: modprobe tun before you start.
usage: airtun-ng -a <AP BSSID> -w <key> <interface i.e. ath0, mon0 etc.>
This will create an interface called at0.
Leave the command shell open and open a second one
usage: ifconfig at0 up
Then fire up Wireshark and start capture on this new interface and voila, you should be able to read the traffic on that AP in realtime.