I've spent a lot of time trying to get number of working AP registered on Cisco WLC 4400 series. Since there is a problem to make SNMP walk sensor (and, what is more important, i do not want to waste licensed sensors for every AP i need to watch :) ), i've created simple .exe file, which return number of AP's, using snmpwalk utility.
This will work if you have Windows.
1) Install net-snmp from http://www.net-snmp.org/ on probe to default destination.
2) Download https://dl.dropboxusercontent.com/u/1209716/PRTG/PRTGTest.7z 7z archive contains .exe file and C sharp original files. Be sure you have .NET Framework installed.
3) Place .exe to PRTG custom sensors folder
4) Make new EXE/Script sensor with this .exe file