You must be on a Release page to add videos to that Release. You cannot add videos directly to a Master Release page. You need to go to one of the Release pages from the Master to add videos. Videos added to one Release within a Master will automatically be added to the pages of every other Release within that Master Release, and all videos will also show on the Master Release page.
If there are no videos on the Release page you will see a link in the YouTube widget, 'Add a Video'.
If there are already some videos you can add more or edit the existing ones by clicking 'Edit'.
Both the 'Add Videos' and 'Edit' links take you to the 'Video Selector' page for that Release, where you can search directly on YouTube. The field is automatically filled with the artist and album name, but you can search for anything.
TIP: If you can't see the video in the search results, but you are sure it exists you can search for it it using a search engine and paste the full URL in the search field on the Video Selector page.
Click 'Search' to see the first 20 results from YouTube. Click 'More' for the next 20.
To check if the video is the correct one you can click the '▻' symbol on any video in the search results to start playback.
Click the '+' symbol to add it to the Release page.
All videos added to the Release are listed in the 'Current Videos' pane on the left of the screen. Once you have found all the videos you want to add you can order them by clicking and dragging videos in the 'Current Videos' pane.
To remove a video, click the 'X' on the video in the 'Current Videos' pane. All removed videos are listed in the 'Removed Videos' pane, from where they can be added back to the Release if desired.
Once you are happy with the selection and the ordering, click 'Save Changes' and you will be taken back to the Release page.