To find your RapidWeaver 6 addons, go to the "RapidWeaver 6" menu item while holding down the "alt" or "option" key and you will see a "Reveal Addons Folder" in the menu.
Copy all of these addons over into a Folder in you Documents folder. For example, I keep mine in "~/Documents/RapidWeaver Addons/"
After you've done this, launch RapidWeaver 8, and go to the "Addons" tab in the "Preferences" window, and choose the the place where you copied your addons to. Close the Preferences and RapidWeaver 8 will prompt you to restart.
When you launch RapidWeaver 8 now, all your addons should be installed and available.