well.. im gonna try to put in the easiest way possible but i find it very hard to explain
lets say dumbo its a model with 3 separated groups
ears, body, eye iris
then when you downloaded the model it came with 3 separated img files, the texture ears, the texture body and the texture eye iris
when you export the model to unity, it automaticly create 3 materials in the material folder, the name of lets say these tree materials are : mat body, mat ears mat eye iris , you found then 3 in the material folder of unity (you can also see this folder as a blue folder name material on the inspector screen while you are editing your game)
if you click in one of these 3 materials.. you will see they have a slot saying "2D texture" so thats where you need to place the corresponding 2d texture, you will have to drag the img over there, or, click on the search button and find the texture by yourself, dont worry as long as you already imported the model and img files to your game's asset folder, you will find it easly by the name
so you have to assign the ears img to the ears material, the body img to the body material and the eye iris img to the eye iris material.. and after that the dumbo will be texturized, drag him to the game just to check if the textures are all in place.... also you will see a lot of shadows, you can fix that by adjusting the shaders
if you still have problems.. try here
http://forum.unity3d.com/forums/10-Unity-Support, i aways go here
or you can add me on msn warpce5ar@hotmail.com
and please dont put dumbo on your game, put kratos, or someone from resident evil =D