When the tab is there, the electricity will run through the entire socket. So, in order for it to work with a light switch, the tab must be broken off to interrupt the hot wire. In the picture below, the the hot side is at the bottom. You might be able to locate the word "hot" directly above the left-side brass-colored screw.
Then, use needle nose pliers to wiggle the tab back and forth until the tab comes off.
Voila! You may then proceed to the outlet installation.
And the finished product: