@ theiza27 & yohji:
The level shows up, when you go to choose one from your list of installed levels to play, as '()' instead of 'Grey (Purple Pineapple)' because the Name and Author fields were blanked in the 'World properties' screen in the editor (or in the world.ini file -- same effect) after the level was created. (In the editor, it shows up in the list of installed levels as 'Purple Pineapple - Grey' just as it should, and the filename is 'Purple Pineapple - Grey.knytt.bin' to match.) So the filename & world folder name tell the game engine where to put the level in the listing, alphabetically, even though we can't see the author or title displayed there.
I was actually wondering how that was done when I played the level, and just taught myself how by looking in the editor. I'd seen levels with a different title in the level-select listing than the name listed in the level archive or used in the .knytt.bin filename before, but it never occurred to me to just blank those fields.
PP also gave the level a blank icon, which makes it look even weirder in the level/world-select screen.
Anyway, theiza, like yohji said, all you have to do is click on or near that () that looks like an 0, and you'll be able to play it.