After accessing ScreenCloud (instructions in the article Create and Manage Sign Content (ScreenCloud)):

Access the YouTube App

1. Select Apps on the left menu
2. Search for YouTube in the search bar at the top of the page
3. Select the YouTube app from the search results
4. Select the "Get" or "Go to App" button (text will depend on whether you've used it before)

Create a New Instance

1. Select the New Instance button
2. Name your instance at the top of the page (e.g., Department Video)
3. Enter your video link in the YouTube URL field
4. Adjust settings as desired
5. Save and Close

Add it to Your Playlist

1. Select Playlists on the left menu
2. Select the playlist you'd like to add it to 
3. Select Add Content
4. Select Apps, then YouTube
5. Select the YouTube instance you just created
6. Click the Add button at the bottom
7. Publish your playlist