Just above the field where you enter text for your post you should see “Upload/Insert” followed by a set of 5 icons.
Now click on the “From URL” link. Here you can paste in the URL to the youTube video you want to embed, then click on “insert link” and that’s it, the video should then be embedded.
For more options click on the “YouTube instructions” and you will be taken to an explanatory page. There’s also a video at the bottom of the page explaining all this.