@Veritas_117 While the link he posted is for a skin, the author of the skin also posted a model that uses that skin. It's in the first post.
@DaneTheBeast The problem is that you changed the texture of the original Revenant model - that's what the Reventant.blp file set to path unit\Creeps\Revenant\ does. So currently, if you chose revenant model, it should display as the air revenant.
In the post where the author uploaded the AirRevenant.mdx and AirRevenant_Portrait.mdx files, he writes that the path is "AirRevenant.blp".
Now the problem is that there is no .blp file named AirRevenant, so either the author made a mistake, or he wants you to rename the "Revenant.blp" file to "AirRevenant.blp".
Either way, the path should be changed from "unit\Creeps\Revenant\Revenant.blp" to "Revenant.blp" or "AirRevenant.blp".
Also a note: When you change path to the skin, save your map and restart your World Editor. Changes like these usually happen after shutting down WE and starting it up again.