Here's how:
Find the vid on YouTube that you want to put in your post, and click to watch it. Then, you need to copy the video ID from the video URL as seen in the following picture:
Then put this code between '[ YouTube]' tags as such:
[youtube]ZCiTKXj5uFg[/youtube]
Which result in the video being embedded into your post like this:
ZCiTKXj5uFg