According to the BioWare Blog, the DLC may be unlocked after you stop the Cerberus coup attempt, but before you attack Cronos Station (thus starting the end game).
Guillermo del Toro, a native of Mexico whose previous features are "Cronos, " "Mimic, " "Blade II" and "The Devil's Backbone, " has said that "Hellboy" took half a decade to make.