I love this sites resource section, it provides you to a link that tells you how to import that particular model you are attempting to get. For the model you used as example, the 'not sure how to import this model' link takes you to this page:
How to import the model NazgulRider
I bet the problem you are having is the area where:
"double click the box "Use Custom Path" and type in the correct path:"
You need the following name:
Textures\NazgulRider.blp
Instead of the "War3 imported map......" name.
Mind this is an auto-generated link, thus it covers both doodads and units. You do not need a portrait image for units (that face image in the interface, or the talking head in cinematic). Yeah most of us who first did this hunted and hunted for portrait image under units. LOL
What ever your project is what ever you are thinking about doing, put it on hold.
I strongly suggest you download unprotected maps, campaigns and the like - there are many here. open them up in the editor, and look at the triggers and how things are done by the author.
You may do anything you like to an unprotected map EXCEPT upload it as your own. Thus you can go through the triggers, modify them, experiment and test the map to see what happens when you do whatever you do.
I assure you that the lessons you learn in this manner are, albeit hard won, but worth the effort, time and energy you spend.
In all honesty if you do not know how to change a melee map, remove the victory conditions, town hall spawning and what not - your no where's near ready to start a campaign map.