TO install/use a model click on the import manager,import the model you want (By the way extract the zip first) then go to object editor,edit model file to Blabla.MDX,restart the editor,it should load the model anyway full tutorial here:
http://world-editor-tutorials.thehelper.net/models.php
P.S. i forgot to tell you that dont panic if you see a green box when you put the unit,it is not properly loaded yet,thats why i asked you to restart the editor...But if you try to test the model ingame,it will work anyway,just not seen on the editor.