There should already be a tree in your back yard on the left. If you click the decorate button and then click on the tree, it should change to the new one. If you bought more than one, you keep clicking until you see the one you want. You can only change the tree, you can't add any.
Hmm thats strange, the tree(s)
should already be in your backyard.
Try logging back out and in again.
If that does not work I would try
contacting Ganz about the problem,
as it may just be a glitch. Good luck!