You will need at least the following:
.1.3.6.1.4.1.14179.2.1.1.1.38 wlan associations.
You may also need wlan interface 1.3.6.1.4.1.14179.2.1.1.1.42
and wlan ssid .1.3.6.1.4.1.14179.2.1.1.1.2
I migrated from MRTG to Cacti years ago and no longer build mrtg scripts.
There is a predefined WLC template for Cacti
See attached image for an example