EDIT: this post from the Stacks Support Desk will help you but if you want some pictures then follow along 

Having issues with installing Flowbox 1.5 or any of our new stacks

By default Stacks 1.3 owns all stacks 

This is what you would see if the stacks add ons are owned by the Stacks Plugin

Double clicking will activate the Stacks 1.3 installer.

However, we are aware of cases where all you see is this

 No problem just copy the two folders ending in .stack so in the case above  'flowbox light 1-5' and 'flowbox dark 1-5' and copy these to 

 /Users/yourname/Library/Application Support/RapidWeaver/Stacks

restart RapidWeaver.

The new stacks will be visible in your Stacks Library View