In order to unlock modern Riddler (not Riddler 1966), you have to complete all the quests in the Hall of Justice/Hall of Doom area. The last one has the Riddler solving a couple color-coordinated switches, after which you'll be able to unlock him. Once this is done, switch to The Riddler and take him into the trophy room at the Batcave.
(copied from the xbox 360 guide)