Every variation's name needs to end with a number, starting from 0, e.g. you can have MyModel0.mdx, MyModel1.mdx, MyModel2.mdx.
In the object editor you set the Model field to the first variation, and the Variations field to whatever number of variations there are.
Although you might have to set the Model field to the custom path of the variations with no number, e.g. MyModel.mdx, or perhaps MyModel, I am not sure, you'll have to check.
There is also the Has Lightweight Model field.
This allows you to offer additional models that are not animated, as an optimization.
E.g. the game has LordaeronTree0.mdx, LordaeronTree0D.mdx, LordaeronTree0S.mdx, etc. for the rest of the variations, where the first one is normal, the D one is dead (e.g. a tree stump), and the S one is for living trees that aren't interacted with.
I don't know however if these will work for any user-made models.