Look for Dr. Secord in the Institute's SRB department. She wants you to find a captured Synth and mark it with a tracking beacon. Your Pip-Boy will display its randomly selected location.
Prepare to face a number of enemies on your way to the Synth. We ended up fighting Raiders. Killing them is optional, but you might as well take them on for the experience.
With all resistance out of the way, you can attach the tracking beacon to the Synth and let someone from the Institute pick it up later. Return to Dr. Secord to complete the quest.