You could ask Zynga's Customer Support for help, but this years holiday tree was probably removed from the market with the last update (Monday, December 24, 2012 at 5:00:00 AM UTC).

I think it was removed today as well, because the icon in the right column for it is gone too. If you didn't get it, that was the way to get it.

Today is also the day you can open the presents for it, so that's probably why its gone now...