I think it's in 5-5... whichever one had the volcano that you freeze first.
It's in one of the spots where you CAN cross the lava river by jumping on blocks, but you can take a high path in the background too - by paperizing a small cave away and replacing it with blocks you can jump from. Go left after you jump up.
3DS Friend Code: 0903-2770-3378. Check profile for more. R.O.B is married. He plugs his Memory Stick into a USB port every night. ~wizaro1