You'll want to click on the 'Scan for Games' button to have the program try to located any compatible games you have installed. While it may take time right now, it'll serve you well in the future. Don't worry, these steps will make installing all mods in the future easier too. You'll need to do a couple of things before you get started. While you can download the files right over here, drag the files into your Data folder to install it, there are also mod managers out there that help solve it like Vortex, which you can find here.
Installing mods is as easy as can be, and Apocalypse is no exception.