M-18 "ScatCat" Remodel
M-18 Hellcat Remodel with XM-551 turret (Milkym4n Remodel) and gun
Simple method: Copy / Move the <vehicle name> .wotmod file from the archive (zip) to the World of Tanks / mods / xxx folder, where xxx is the current game version
Better method: Create a folder in the World of Tanks / mods / Xxx folder, where xxx is current game version, named vehicles. Then move the <vehicle name> .wotmod file from the archive (zip) into the that vehicles folder. This keeps things a little more organized.