07gtcs ------------;
Congratulations on your new PC !
The wrench that fits on the flat nut behind that backing plate came in the bag with the instructions, did you find it?
The way to use it is to get the wrench attached to the flat nut on the back of the backing plate, then hold it in one hand, and turn the backing plate counter-clockwise, with the other hand, and it will break loose, so you can unscrew it from the machine.
i would recommend you get the 4-15/16" backing plate for the 5" pads, as the pads need to rotate to work best, and the smaller diameter will allow this easier than the larger diameter pads. When you get the hang of it, you can even get 3" and 4" backing plates and pads, to allow you to work on smaller areas easier like tail lights, etc..
The correct backing plates will have a velcro covering on them that will adhere to any foam or microfiber pads you get from this site, or other sites, etc...
Good luck ! Enjoy your new tool !
Dan F