Hmm... ok, well, as i've never actually used a custom model, this may not work... but.... Ok, you import it, then you make a custom unit, or use a regular one, dont matter. Then find the field that says Art - Model, and change the string to whatever the path of your imported model is (war3imported\...\...) and it should work
In the Readme that came in the zip that you downloaded, is says where to put everything. After you import everything from the zip (except the TXTs) using the Import Manager. Then, right click and select 'Use custom path' or 'Modify somethingorother" and change the path to the one specified. You have to save, restart the editor, and load before it will show up.