Note - Your video must be uploaded to YouTube or Vimeo in order to be attached
#1 - Go to your listings section
Go to your listings via "Manage Listings" tab at the top of your backend, then click on "Listings".
#2 - Find the Listing you want to add a video to and click on "Attach"
#3 - Paste & Link
Depending on where your video is hosted, go to the "Youtube Video" or "Vimeo Video" tab. Now you can copy-paste your link in the box, and click on the blue button to link the video.