Acutally there is a way to skip a level.
Control-click on Marble Blast application. Choose Show package contents. Now go to marble/client. Find a file called prefs.cs. Open it and look for something like $pref::QualifiedLevelAdvanced(Beginner,Custom,etc).. If you find 15 change to 16,20 to 21 and volia! You've skipped the level!