You can embed a YouTube video in a post by following these steps:
1. Open the Advanced Editor (blue circle)
2. Click on the Insert Movie icon (red circle)

The Insert Video Clip Dialog will open (see below).
3. Enter the URL to the YouTube video

4. Click OK
Now you will see the code for the video in the Post editor.

When you SAVE the Post, your video will appear as in the example below:
1. Open the Advanced Editor (blue circle)
2. Click on the Insert Movie icon (red circle)
The Insert Video Clip Dialog will open (see below).
3. Enter the URL to the YouTube video
4. Click OK
Now you will see the code for the video in the Post editor.
When you SAVE the Post, your video will appear as in the example below: