How do I access the additional content included in the Definitive Edition?

Most of the DLC packs included in Sleeping Dogs™: Definitive Edition are enabled by default: additional missions, outfits, vehicles, weapons and items included in the DLC packs can be played or found during the main story mode.

The Nightmare in North Point and Year of the Snake DLC packs add new stories to the game, which are separate from the main story mode. You can access them via the Main Menu.