You can get a bus from Fethiye to Dalaman, its any of the buses that go to Marmaris from the main Otogar or you can flag them down on the main Dalaman Rd. However be aware if it's the airport you want then the bus station is quite a way from it. I believe the Taxi drivers do tend to have you over a bit for the short ride.
If it is the airport you want, don't bother messing about with buses, you can get private transfer with Pegasus for £30 for up to 5 people.