For dynamic mesh you need first to "isolate" the zone in gambit, basically you define the zone and put non-comformal faces. If you use MRF instead of the dynamic you dont need the non-conformal face.

Then in Fluent when you define the boundary you can define the rotation of the fluid-zone with MRF or Dynamic mesh. If you use non-conformal mesh you need to declare you interface-grid. Then if the profile of the rotation is not steady but variate with the time, you can write a UDF to define the angular speed.

(from CFD-Online)