I have a problem on level 5. There is supposed to be a portal in the big red dot that takes you to the secret level, "Who Turned Out The Lights", but jumping into the dot has no effect…

This spot is top left of the level where you kill the three Professor Monkey-for-a-head enemies who open the valves that release the floating eyeballs.
I've restarted the level multiple times and I still can't access the hidden level.