You have to get a taxi from Dalaman bus station to the airport. It is about a 10 min drive (or under) and will cost about 20tl. Ask the price before you get in a taxi - they must be amongst some of the most expensive in Turkey!
I am sorry to say there is little choice, if there is a Havas bus God knows where you would pick it up or when it goes. The only other option would be to hitch hike, should be easy as nearly everything past the main roundabout is going to the airport. Mind you the only people I have given a lift to have been Policemen? Well they have guns don't they!