You have a few options for adding YouTube videos to different areas in D2L, such as emails, discussion posts, content topics, etc... 

 Use the "Insert Stuff" button in any D2L area that includes a text editor. 

When the "Insert Stuff" window opens, click on YouTube in the menu at the left side. Use this window to search for your video. 

Click on your video to select it and click the "Next" button at the bottom left side of the window. 

The window will refresh to show a preview of your video. If it looks correct, click the "Insert" button. 

You will be taken back to the text editor window you started with. Here, you can finish editing your text and save/post/send your document.