Instructions
Start any game mode in "Brawl" and wait for the character screen to load.
Select Link as your character by hovering the pointer over his head and pressing "A."
Press the "Y" button on your controller until Link's coloring changes to that of Dark Link. If you are using the Wii Remote, you can press left or right on the directional pad instead.
Press "Start" to begin the match. You, much like Dark Link, will have all of classic Link's abilities, but with a much more sinister appearance.