To generate the mesh, click Solve on the Solve dialog. The mesh is constructed according to the prescribed element distribution. If a mesh already exists (and you are simply continuing the analysis), then a new mesh is not created.
To view the mesh prior to running the analysis:
This is a good opportunity to check the mesh to ensure that it resolves the geometry adequately.
What if I don't apply any Mesh Sizing?
Autodesk® CFD provides a completely automatic mode of mesh definition. If you do not define a mesh distribution, Automatic Sizing is automatically applied when the simulation is started. The process is automatic and seamless.