When you open a purchased download of Stacks from us, you'll receive a folder that contains at least one (but in some cases) several .stacks files. To install, simply doubleclick each .stacks file. These files should have an icon that looks like a lego brick. If you see ONLY generic folder icons and NOT lego icons, you have something amiss with your installation of Stacks or RapidWeaver. First, make sure you have *the* most current version of both Stacks and RapidWeaver installed.
NOTE: As of RapidWeaver 5.3.1, RAPIDWEAVER handles the stacks installation process. Due to Apple's continued sandboxing efforts with App Store applictions, from 5.3.1 on, any time you click on a .stacks file, RapidWeaver should launch and handle placing the .stacks into the appropriate folder.