Click on your 'add paper scrap' button and then in the bottom left of the window that pops up, click 'Browse...'.
Now I'll give you instructions on where to find the files... this is where I find mine and your's should be similar.
Click on 'Gallery' on the left side
Click on 'move up one folder' button - located at the top right of the window (it's a small image of a folder with a green arrow)
You should now be in a folder called 'images'
Open the folder of the kit where your background comes from and browse through the files.
Double click to select the background you want to use.