1. Yes. Trigger it.
2. Click "Import".
Well... lets not be a total prick.
You can create many custom effects and add them to a spell.
In the object editor you can find many fields where you can fill in special effects.
But when you take a look at the trigger editor and learn a bit about casting abilities and stuff, you can see that you can do almost everything without problems.
It all depend on what custom effect you want and when you want it.
To edit models in the model editor... at least that is what I understand, you have to export the models from a map or download them somewhere.
Then you can open that file in the model editor that you use.
Warcraft World Editor has no in-built model editor.
What you might have meant too is how to import models into your map.
You have to go to the import manager and import the models.
Then you go to the unit or item or doodad or whatever and place that imported model in the art field of your object.