Active hacks will appear in the bottom left corner of the screen in Adventure Mode (See How to Equip Different Hacks for information on how to select which hacks are active). To use a hack, press the number of the hack you want to use (or scroll through them using the Mouse Scroll Wheel) and aim the reticle at your target. An object that can be hacked will appear with a pale blue outline when hovered over. Press the Right Mouse Button to fire the hack.
Hacks use energy, which is represented as a blue bar to the right of the active hacks. The energy cost of each hack depends on the value of its effect (for example, the more health the Health Change tweak is set to restore or remove, the greater its energy cost). The energy cost and parameters of a hack can be viewed and altered in the Inventory.