Really it can go anywhere in the inlet tract after the turbo or s/c. However, best is to have it just slightly down from the throttle body - pointed at the throttle body. Means its far enough away to mix well with the air, and also atomise better if it smashes off the throttle plate. You want it pointing with the flow of air obviously.
Not a great picture, but you can see where the 5th and 6th injectors are on my old setup, the beige looking things next to the master cylinder.