Not sure if you have to beat the game to make it show up, but from the world select screen look to the left and you'll see a new world to select, the Helladeck.
There are 50 levels in the Helladeck, in order to beat them you must collect all Glorbs in a level. Completing five of the levels will earn you:
Some of the later levels are probably harder than the levels found in the main campaign, but they shouldn't really cause too much trouble. Some of the earlier levels limit how many times you can rewind, while the later levels are really lenient with how many times you can.
The one level I did have a bit of trouble with, was the one where you can only play as Zackasauras. I didn't realize for a while that you can bounce on rockets and enemies, so if you think you're stuck you need to bounce on stuff to finish the level.
Thanks to Dire Nevermore for posting lsdMyth's video