Once I figured out how it was supposed to work (no documentation), it was pretty simple. Simply publish the module in any position on your home page or any site page, and make certain it is completely enabled. When a visitor comes to that page, the Age Check form will pop up with a colored "veil" hiding the rest of your site. (You can set opacity for the veil.) If the visitor enters the right date to meet the age check, the visitor is directed to your site page. If the age check fails, the visitor is redirected to a url you specify.
On the other hand, I can't use this module. I need to customize the look and feel a LOT more than the module allows. The way the php files are encoded, I can't even make my changes there.