From the big road in front of the airport you can get the bus to Inzgan (suburb of Agadir). Just wait at the other side of the road and give a sign when the bus is coming (5 dh). While you are waiting sometimes the taxi drivers beside propose you good prices for the ride to Inzgan.
In Inzgan you take a “Grand-Taxi” (6 persons) to Tiznit (35 dh) to continue to Mirleft (13 dh). There is also a bus coming from Marrakech in the afternoon which is going to Mirleft.