The old mapster used "<<youtube ######>>" where #### is the unique video ID and it would automaticly add the video to the post. This was the -> Wikicreole <- code.
The new mapster uses -> html <- format which is more efficient and customizable. The new text box which you use to reply generate the code behind the scene when you change formating like bold, underline, links, text color, etc. Doing the old <<youtube ###>> code will not work. In fact, in order to do coding into this text you can't do <strong>this text is in bold</strong>. You would have to go into the "Source Code" which is a button that looks like this "</>" In there you will see the coding of your message and be able to apply changes manually.
Go to youtube and find the video you want to integrate. Under the video, press "Share", then there is an "Integrate" button which give a line of code similar to this: <iframe width="560" height="315" src="" frameborder="0" allowfullscreen></iframe>
Go to mapster where you want to reply, press the Source code "</>" button which open up a text box in which you can code. Copy paste the line of code from youtube and press Ok. It should now pop into your message and when you will post it will show up as a video. You can scale it if you wish. You could probably change the -> video code <- to do other stuff like loops.