You need to remove the valance (covering over the outside part of the dash). Once removed you will see the air inlet and space for a filter. It is a rectangular shape a bit smaller than a standard air filter. The smell you are experiencing is likely to be as a result of the water not draining away from the condensor (you should see water dripping from underneth the transmission area of the car once you have parked it - after having used the a/c). Mould has very likely started to grow hence the smell. If not removed it will only get worse. The pollen filter will not reduce or prevent such odours getting into the vehicle. You need to identify the source/cause of the smell and then go about cleaning/removing it. Could be a major job though.
It is possible to clean the drain outlets by getting under the car and in the area of the transmission (usually r/h side) you should see a black rubber tube pointing downwards, it is a couple of inches long. Sometimes by gently inserting a piece of wire and jiggling it around you can clear any blockage. Just make sure you don't have your face directly below the outlet just in case it contains dirty shitty smelly water. Good luck.