This is the most common misconception. If you search "PowerMill MTD file download" on Google, you will find hundreds of sketchy forums and third-party websites promising free files.
Ensure you are logged into your Autodesk account within PowerMill (Help > About > Manage License). powermill mtd file download
If you own a less common machine (e.g., a 5-axis EDM or a custom gantry router), the builder may have provided an MTD file on the installation USB stick that came with the machine. If not: This is the most common misconception