There are a couple reasons the MDL Loader might not work.

Unsupported Versions of Max

The MDL Loader will only work in 3ds Max 2015+. If you need to use the MDL Loader, you must have 3ds Max 2015+. Currently the MDL Loader works in 3ds Max 2015-2017. There are no plans to support older versions.

System Paths are Not Properly Configured

In Wall Worm 2.944+, this problem should never happen since the functions to set those paths are now run automatically in Max 2015+ when they haven't been set. If that doesn't happen in later versions of WW, you can run this MacroScript in the MAXScript listener: macros.run "wallworm.com" "WallWormInstallWWPaths"

During the installation of WW, the installer should set up to paths for loading plugins. The exact path is dependent on where your version of WW is installed and what version of 3ds Max you are running.

So the best solution is to make sure you have the latest version of WW.

If the installer failed to add these paths, you can manually add them. To do so:

1. Click Customize > Configure System Paths...
2. Open the 3rd Party Plug-Ins tab
3. See if there is an Entry named "Wall Worm Common" that points to a startup folder in the WW installation path.
4. If there is no entry, click the Add... button and browse to this folder: PathToWWInstall/WallWorm.com/startup
5. If you did #4, enter the label "Wall Worm Common" and click Use Path.
6. Now look for an entry that is labeled Wall Worm VTF that goes to a path like assemblies/20##/x64.
7. If there is no entry, click the Add... button and browse to this folder: PathToWWInstall/WallWorm.com/assemblies/20##/x64
8. If you did #7, enter the label "Wall Worm VTF" and click Use Path.
9. Restart 3ds Max.

Note that the path named Wall Worm VTF is labelled as such because it is the same path WW Pro uses for the VTF plugin and is reused for any future plugins being added to WW like the MDL Loader.

Also note that the paths used above that include 20## require you to choose the version of 3ds Max you are using.