There are two ways to add video to your post: by linking to the video site, and by embedding the video here. The embedded option is preferred, and is the default option. To embed a video in your post, paste in a link to the video and press "Enter". For example, to embed this video about cooking a tortilla, paste the video URL into your post:
After pressing "Enter" you will see:
To simply link to the video you must give the link text first, for example "Cooking a Tortilla" and then select that text and click the "Add Link" button in the toolbar. Paste your URL there, which will give Cooking a Tortilla.