I've been stuck here too, for like an hour; I finally caved in and looked up the answer in Keyblade's guide (it was a little hard to fully understand what he was saying at first...). Go to 3-5, and there's a hidden block inbetween the first two islands. You can use that to get to the center island.
The Stickman Chronicles Swapnote comedy series: PM me your 3DS FC if you want it! More Info: http://tinyurl.com/6rmk8xb