It is possible to install stacks in different ways.
After purchase and download of a stack, you will get a zip file. Unziping this file will show you a productname.stack file. There are 3 different ways to install that stack.
Double clicking the productname.stack file will in most of the cases install the stack in the correct RapidWeaver add ons location. It might fail if you have multiple versions of RapidWeaver installed.
A better way to install the stack is by dragging and dropping the productname.stack onto the RapidWeaver application icon. With this, you make sure the correct version of Rapidweaver is installing the stack.
Opening the RapidWeaver preferences menu will let you open the add ons directory inside Finder. Copy the productname.stack manually into the Stacks folder.
Restart RapidWeaver always after installing a new add on.