To make a link active, call it whatever you want in the text, like, "link" if you're feeling super creative. Then highlight the word(s), and the chain buttons in the task bar above will light up. Click the chain link button and you'll get a new window. Put your link's URL OR your YouTube video's link in there, and then choose whether you want it to open in a new window or the same window. Then hit ok - your link is now active. You can test it in preview before posting. If you messed up, highlight the term again and hit the button that looks like a broken chain link and do it again.
Second way to embed a video: Copy the URL of the video, then in your post put: View:http://www.whateverwhatever.com but put [ ] around it.
Thanks sweetiemuffinsugarplum for the tip.