This is almost always a pathing issue. Ensure the MOLDWIZARD_DIR points to the folder containing the templates and parts subfolders.
NX looks for a specific variable to know where the library lives. If this isn't set, the "Mold Base" command will return an error. Nx 12 Mold Wizard Library Download
The library usually contains both. Ensure your NX part file units match the library units you are trying to load to avoid scaling issues. Pro Tip: Customizing Your Library This is almost always a pathing issue