Intrusion Detection System (snort) - Airport Extreme (802.11n) Base Station

I'd like to run an intrusion detection system (Snort -- http://www.snort.org/))
on an Intel iMac connected (via one of the 3 ethernet ports) on an AEBS 802.11n.
This base station is set up to participate in a WDS network (as "WDS main"
communicating with 3 Airport Express units) and as a router connecting to an 8 port
netgear switch (to which various computers/devices are attached) (and, to the
already mentioned iMac). It's WAN connection is to a cable modem box.

The AEBS is working fine. Snort is working fine on the iMac - but it sees
only traffic to the machine it's hosted on. I need it to see all network
traffic to/from (and within) my local network (i.e., via the AEBS).

I guess the AEBS functions as a switch - sending only traffic destined to
each address. Can I configure it (or the port on which the iMac is connected)
to see all LAN traffic? Or are there any recommended alternatives that would
permit me to do this (with other network gear, etc.)?

Thanks for any suggestions/help.