It is a two step process to mesh a mold. First you mesh all surfaces (mold, mold boundary, cooling channels, inserts) with triangular elements, and then you mesh the volume of the mold with 3D elements based on those triangular elements.
Ensure that all layers are selected.
Click (Mesh tab > Mesh panel > Mold Mesh).
Select the Surface tab within the Tools panel
Enter 16 mm for the External mold surface edge length.
Enter 4 mm for the Internal mold surface edge length.
Enter 12 for the Target number of nodes on circumferences of cylinders.
Click Mesh, to launch the Simulation Compute Manager.
In the Simulation Compute Manager, select the solve location, then click Launch.
When the mesh is complete, uncheck the Mold External Triangles and both Channel layers.
The surface of the runners and cooling system are now represented by triangles.
This will take several minutes.