If that’s the only problem you are having then I think deleteting and reinstalling everything may not be needed.
How did you install the stacks? Try either “dragging” them to the RW8 icon on the dock, or right click on the stack to be installed and select the Open With then choose RW8:
I don’t know anything about the app store version. but RapidWeaver 8 (and Rw 7) allow you to “set” the location of the addons Directory in pwefenences.
Also If you reveal the add ons folder RapidWeaver 8 You will find a directory called stacks. That is where the stacks should be installed.