This very small stereo microphone set plugs into the hot shoe powered accessory port. All power for the microphone comes from the camera, though the drain seems to be minimal for this accessory. The reason why it's called and "adapter" and "set" is that the actual widget that plugs into the port is just contains some electronics and a 3.5mm microphone jack (the adapter part). Olympus includes two microphones that plug into it: a very small stereo hammerhead (left and right mics point left and right), and a cabled, clip-on lavalier microphone. Both seem to work quiet well, though note that the hammerhead is not isolated from camera handling noise. So basically this accessory gives you three options: (1) a small hammerhead that'll sit just above the camera; (2) a lavalier that you'd likely clip onto yourself as you're using the camera (for narration, I suppose); and (3) any microphone you've got that uses a 3.5mm jack (though there's no place left on the camera to mount said microphone, as the SEMA-1 is using the hot shoe).
Thom's Mini Review: If you're doing video with a Pen that has the powered accessory port, you'll really want this accessory. It's small enough to carry in any pocket, it comes with two decent microphone options and a jack that supports more, and it really does provide better audio capability than the cameras get by themselves. I haven't found there to be any real power consumption issues with the SEMA-1, and the hammerhead mic gets decent stereo separation at the camera position without being too directional (it does seem to have a modest weak spot directly ahead).