Originally Posted by Cully
Pretty sure the last one is unlocked when you have gold on all others.
OR.. Have you picked up the Ceramic Rooster in the Laboratory? If not... go to the Laboratory and theres an Enigma Keypad down a small stairway. Inside is a Ceramic Rooster which unlocks one of the Challenges. This is the one most people miss.
As far as I know, you only need the collectibles (incl rooster), and then golds on previous challenges to unlock them all. I didn't have to do anything special. If you have everything else, it's very strange for 3 challenges to be missing.
Just to clear something up, I unlocked every challenge before even starting the second one, so getting gold on all previous 9 is wrong.