How to add a character to a scene I've downloaded on Asset Store?

Well, I downloaded a scene from the Asset store and now I want to make a game by adding characters to it(also downloaded from the store) but when I drag & drop them to the ‘scene’ part I can see them, and then when I switch back to the ‘Game’ mode, all I see is one part from the downloaded scene, I can’t move nor I can see the characters I’ve previously added. Please help me. Thank You.

You need to add a character controller to your scene, then add the character to the controller object. There are several really good demos on how to do this in the tutorial section.

I would also suggest downloading the Unity Sample Assets and playing around with that demo.