In the first chapter (Hong Kong Marketplace), head to the Ambushed section. After you kill everyone in the courtyard and move along the alleys, you will be forced inside a building (there's no where else to go). As soon as you enter, there should be a door you can break. Break it and go along the corridor until you reach a movie theater. On the movie screen are trailers for Wheelman and Blacksite.
To unlock a harder difficulty that is originally unavailable in the game, complete the game once through on Casual Mode.