DIAGNOSIS It's tucked up inside the vent chamber and isn't meant for frequent replacement unless you're in recirculating mode. People often miss it because it's not obvious.
FIX First, open the vent grille on the underside of the microwave (the exhaust opening). Look all the way to the back of that chamber, up near the top. You'll see a rectangular frame. Push up on the tab at the front of that frame, then pull the whole filter assembly straight down and out. The charcoal filter is a flat, mesh-like panel that slides out of that frame.
IF THAT DIDN'T WORK If you have the model with a slide-out grease filter in front, sometimes the charcoal filter is behind it. Remove the grease filter first and look inside that slot.
HEADS UP Only replace the charcoal filter if you're using the recirculating (non-venting) mode. If your microwave vents to the outside, you don't use the charcoal filter at all, and a missing filter will actually improve airflow.