All A/C systems produce water (condensate) as a by product and that water must have a place to escape. My guess is that the hose that attaches to the housing back behind the dash is probably not draining well and is therefore forcing water to collect inside of the housing. As the water sits it forms mold/mildew - that's what you are smelling. You're going to need to get that drain opened.
Your other symptom - no cold air at idle, indicates either the A/C system is low on refrigerent or that the compressor is weak.
You may be able to resolve the drain issue yourself - if you are so inclined. The 2nd issue requires a trip to an A/C shop of or the dealer.