"Importing" refers to the process of getting a 3D model (currently in Collada format) from a modelling program into Outerra and script it at all, and that's a bit complex, but there are other people's tutorials in the import section of the forums.
But from what you wrote, I guess you are rather asking about "installing" the mods - that's simple, once you download an otx file, just doubleclick/enter/launch it, it will be automatically installed by Anteworld.