Hello, nebulizing 7% saline or exercising is Step one in airway clearance, it gets the mucus moving. Step 2 is "getting the gunk out" using any number of techniques - using the Aerobika, active breathing, huff coughing, postural drainage, using a percussion vest...
Here is an excellent video that covers why and how:

I am curious as to whether you are taking antibiotics for MAC, and if so for how long?