create any folder named prefab (or a name you'll remember) . load map in radiant , click on file click on load prefab browse to your prefab folder . i have mine in my documents/mohaa modding/map prefabs
When you load a prefab it will be in the same place as it was when the person who made it had it and all the parts will be selected . You will need to move it to where you need it and probably have to raise or lower it . Just make sure you do that before you hit esc to deselect it .