Ok, here goes:
When you want to embed a youtube video(only works embedding youtube videos), follow these easy steps:
When replying to a post, hit the youtube button. In between the youtube and /youtube values, you need to put in the video identifier.
In the case of http://www.youtube.com/watch?v=zKUkRgzCL44, just put the value of zKUkRgzCL44 in between the youtube and /youtube commands. That's it, all set. Thanks for the suggestion Reaper. I had been looking into this for a while before, but couldn't find it. The link you sent was for phpbb2, while we run phpbb3. I did find what I needed though on another thread, and we're all set