its actually very simple. im assuming this is just a already made model u got off the site.
1. you open WE and load your map.
2. then you open the import tab
3. next you click import and select your model, im most cases you don't need to change the path at this point.
4. next save your map and exit it, because the WE doesn't recognize it yet.
5. load you map again.
6. go to object editor and select a unit you want, doesnt matter which.
7. scroll down until you see model art in the editor.
8. click it and select the model you just imported.
9. close out of object manager and open the unit pallet.
10. then place the unit on the map and bingo its there.
there are some cases in which you have to import textures with the model and usually they are Textures/whatever.blp
if it doesn't work just keep trying and maybe look on the site for a tutioral... i think there may be one on wc3campains. hope this helped... good luck.