Steps:
Find the menu item you want to add the media to.
Give permission for the app to access your photo library (if you haven't already)
Click on the "+" icon on the item page.
Select either the image or video icon based on the type of media you want to assign
Select the image or video item from your camera roll.
After selected, you can edit the image if necessary.
That's it! Your media element should now be assigned.