This nice tip is by Adam Shiver of Elixir Graphics:
With the release of RapidWeaver 7 there is now the ability to run two versions of the app side-by-side. You can now have both RapidWeaver 6 and 7 installed and running at the same time. This can sometimes lead to confusion of where a plugin, theme or even stack needs to be installed.
If you’re double-clicking an addon and it’s not being installed in the version of RapidWeaver you wish it to be, give this a try: Drag and drop the stack, theme or plugin right onto the RapidWeaver icon, of the version you wish it to be installed into, in the Dock. Doing so tells that version of the app that it is the one that should be responsible for installing the addon.