The "tree" you need to find for the achievement is placed in the tutorial level, in a hidden high spot.
Since you can't return to this map once you reach the first kingdom (the wood kingdom) you must start a new game (you can choose a new save slot in the menu).
Play the tutorial as usual until you pass the village and enter the woods.
In no time you should find yourself in this place. Stop right there.



The tree is just above your head, you just need to climb that wall on the right.
As you can see the jump is a bit tricky but you shouldn't encounter any particular problem if you do like shown in the following video:


(video uploaded by SK4TE HiGH, I just found it on youtube)

If you followed what was said in this guide by now you should have found the purple "tree", or shape, or whatever that thing is....

Yay you dit it! Congrats!