Have you got the umbrella? If not, remember that pillar with the porcupine in it? Jump to the left to find blocks in the air leading to a sky castle. (If you're already there, if you see houses, you're in the right vicinity.)
If (or once) you do, you're set to follow the rest of Karel's instructions. Travel the ceiling a few screens above Karel to the right. You'll know you're near the entrance by the ghost-passage indicators lighting up the right of the screen. Karel's instructions will be a little tricky to execute (but rather quicker than you'd expect).
Had you got my PM all that time ago? Still wondering if you managed to download those levels.