Due to Mac OS X sandboxing, the addons folder is quite buried inside the file system. But RapidWeaver 6 has a new shortcut to open up the folder.
In RapidWeaver 6 hold down the Option key and choose Reveal Addons Folder from the View menu.
The Reveal Addons Folder item will only appear in the menu when you are holding down the Option key.
The full path to the addons folder is: