For mold block surfaces defined by the Mold Block Wizard, creating the 3D mold mesh is the final step before running a Cool(FEM) analysis.
Make sure you have created a mold surface mesh before you attempt to create a 3D mold mesh.
To convert a mold surface triangular mesh into a 3D mold mesh:
Create Mold 3D Mesh in the Study Tasks pane to open the Generate Mold Mesh dialog.