What do you mean by "Layer AD"? If you mean DIV with AD which will be showed when download page loads - you can easily do that by yourself. Just some HTML and JS knowledge is required. I'm sure there also a lot of samples on the net.
That can be done directly in template. I dont think that we should place bunch of ads in admin settings. You want AD Layer, someone will want popup, then popunder, then survey, etc.
You're free to edit template and put there any type of AD.