Its always the same nooby question and always the same easy answer, but always an easy way to get rep
1. Set right pathes
Nearly always, in the folder you downloaded there is a Readme file.
Open it and find out the right path. Copy it.
Now open the Import Manager in the World Editor.
After importing the model, double click on the imported file.
(or press right click on the file and choose "Modify Model Properties"
Now click into the gap to use a customn path.
Fill in the path out off the readme file and press okay.
Often you only have to remove the "war3mapImported\".
Now it should work, but if you use it the first time, a green box will appear in the World Editor. To fix this, do
2. Re-Open World Editor
After you set the right pathes, you have to close (and save) the WorldEditor and then open it again. Now the model will be shown properly in the WorldEditor.