There are some threads in this forum regarding this already. It is not as easy as monitoring a switch or router. the controllers combine the AP mac with the oid for unique lines. You can glean some snmp info off the controllers by using some of the generic OIDs and controller specific OIDs, but you will need an snmp monitoring program that you can load the WLC MIBs into and then tailor your monitoring of the controller based on that. I use Cacti, which is really awesome, but I would need to grab the complete OID off each controller based on the APs on it, which is fine as long as the AP is on that particular controller and not another one. snmpwalk is your friend in this case.