How to embed a YouTube video is covered in the video below, but here is a summary.
Copy the embed code (shown in the screen shot below). When you edit the Google Map, switch to "HTML" editing mode and paste in the code. Save that and whoa-la, you've got it.
CAUTION: If you edit a posting in "Rich Text" mode that already has a YouTube video embedded in it, you will lose the code for the embedded video. You will then need to switch back to the "HTML" mode and re-paste the embed code to correct it.
YouTube Video on how to use the Google Maps: