To embed, just copy the YouTube link, click the little icon in the posting options, paste the link. If it's a correct link (like the one you posted at the top of the page), the site will automatically take the code and you don't have to type it in manually.
If for some reason that doesn't work, copy the part after the "=" sign and post it in the embedding box instead:
youtube.com/watch?v=-GerpU9L1-I
[YOU TUBE]-GerpU9L1-I[/YOU TUBE] would work if there wasn't a space there.