Hello! The .rpa isn't a game build file, clicking it or trying to open it won't do anything. You have to add the rpa into the main Our Life: Beginnings & Always "game" folder to unlock Derek's content.
1. Download the zip file after purchase and extract the zip to get a folder
2. Open the folder of your current Our Life Build and find the "game" folder inside. To view folders on Mac you might have to right click the application launcher and select "view package contents" and you may have to open a couple different folders before you find the "game" folder tucked away inside
3. Move or copy/paste the rpa file into the Our Life "game" folder After that, the build you already had will now include all the extra content this DLC gives.