Ok, lets start simple. When importing a model its important that you import the mdx and blp(if it has any) files, not the zip. You will then need to change the custom path of the model, if you dont know what it is then just simply read the tutorial for the particular model, every single model on this site has a tut for it. So just read that and you should be fine. Once its imported just go to unit data select your unit that the model is for, then change the model to the imported one.