Stand in front of a tree. When positioned correctly, you can climb it by pressing UP. If not in the correct position, you will jump up and fall down once you reach the top (the falling is slowed if you are "on" the tree as you are going down. You can also do this "jumping" on the stars that appear in the sky in many places.
Don't take the trophy and go through the exit door. Instead, turn to the right side and go to the right door. You will find the warp zone to level 8, where jetpack is also available for you. If you choose the left door, you will reach a warp to level 0.