I forked the above code and came up with a quick and easy jQuery solution that may help. I did jquery because it was, as I said, quick and easy, but this effect could be done with CSS3 transitions too (or close enough).
Hope it helps. Feel free to fork it yourself and play around; you could use icons in place of the plus symbol, make the icon change, make the menu slide back up with a conditional statement… etc. I recommend playing with it, and post it back up if you get stuck. =)