See snort-wireless http://www.snort-wireless.org/ for more information.
ifconfig ath0 down wlanconfig ath0 destroy wlanconfig ath0 create wlandev wifi0 wlanmode monitor ifconfig ath0 up snort -devw -i ath0
Be sure you see beacon from your ap, then configure your snort.conf configuration file (configure --enable-wireless) and invoke
snort -c /your/path/to/snort.conf
NB: Snort-2.4.3 may have some problems.