All Source engine models have a skeleton/armature. Can you upload the files you are trying to import, please?

Can you upload your .blend file, please.

It's trying to convert the DMX file to a version it understands. I still have no idea what the correct version is though.

There must be some other discrepancy between the animation and the skeleton, then. Can you export the entire model from Blender and compile that?

The embedded versions could be out of date, then. Perhaps 5/18 will work? If not, you will have to find a DMX file that will work with that engine build and see what it uses.

Are you exporting your animations with the same up axis as the reference mesh?

Blender isn't providing an important method that it normally does provide. I'm not sure why this is happening, but it might go away if you just restart Blender.

Click on the Help link and you will be guided through the process.

If you provide an engine, path, the exporter will automatically select the correct DMX version for you.

Interesting, I fixed this only recently. I thought it was an old bug, but since this is the second report about it to appear in a short space of time perhaps something has changed elsewhere.

Anyway, you can fix this by downloading a development version of the addon from GitHub, or by waiting for the next release.