Instructions
Complete level the last level of Year 1, "Face of the Enemy," on "Story" mode. Play through the level again in "Free Play" mode until you reach the room with the giant chess set. Use dark magic on the first chess piece in the room to get the Professor Quirrell character token.
Complete the last level of Year 2, "The Basilisk," in "Story" mode. Play through the level again in "Free Play" mode until you reach the fight with the basilisk. Go all the way to the back right section of the screen and cycle through your characters until you reach Griphook. Use him on the keyhole of the vault to reveal the Tom Riddle character token.
Collect all 200 gold bricks. You get these bricks for earning each "True Wizard" status, finding each "student in peril," completing each "House Crest" and finishing each of the 24 levels. Other gold bricks are scattered throughout Hogwarts or can be purchased. Go to "Borgin and Burkes" shop on the right side of Diagon Alley after you've collected all 200 gold bricks. Assemble the pile of gold bricks in this shop to create a doorway. Enter this doorway to play a bonus level and unlock Voldemort. You can either collect one million lego studs to finish the bonus level or exit out of it, but Voldemort is unlocked as a playable character as soon as you start the level.
Go to Madam Malkin's shop in Diagon Alley. This is the first shop you'll see when entering the alley. Purchase the character tokens for Professor Quirrell and Tom Riddle to unlock them both as playable characters.