"To add to that, the easiest way to get your addon folder if you are unsure. Log into rift. At your character screen click the Addons button in the bottom of the screen. In the window that pops up find the button that says Open Addon Directory. It will minimize Rift and open a window that will put you in the interface/addons directory. Good luck. Let us know if you need anything else. "
I was having the same issue, this worked. Thank you!