Ok now what?

I just downloaded a new model so now what? ;/

When you download a model is should come with , goku.mdl or somthing like that just place it in /esf/models/players/goku and over right it , but make sure your over righting the right one

Or if the model is named differently, rename it to the model you want to replace. The models are found in esf/models/player as Superbolt said.
so if you downloaded a model named ssj4vegeta and you want it to replace the ssj vegeta, rename the .mdl to ssjvegeta.mdl and replace it in the ssjvegeta folder.

But it say's rar file what the hell is that?

Open it with WINRar , http://www.rarlab.com/ , its another form of winzip

In other words its a method of compressing the file to make it smaller for file hosting. Once you get winrar, open it and extract the .mdl to wherever you can find it.

Be sure it's 1.2 compatible