Go to your surfaces tab, find which part you want to add it too in the list.  From there, click and the listing of diffuse, ambient, spec, ext. will be below-but in the advanced tab.  You click the arrow to the right I believe to get a pop up menu. Go to "Browse" and find the map you want to use in Ambient and the settings on how much to use will be the dial below it.  Render and see how it looks and go from there!