There are three types of in-seat power adapters. They are largely found on business and first class seats on long-haul or international aircraft. As mentioned above, seatguru will tell you for sure.
The most common is a standard automotive style 12V plug. You can get a 12V DC to DC adapter that is specific for your laptop, or a universal one from Targus or similar. For your mac, there is a Mag Safe 12VDC adapter available for about $50. The second type is the EmPower plug, also 12VDC but an airline specific plug. Some airlines offer 110VAC at the seat.
Here's a picture of the magsafe adaptor with both 12VDC plugs:
As mentioned, you can also get a 12VDC-110VAC inverter.
But the likelihood is you'll be running off batteries. When you open your screen, be careful not to get the top of the display under the tray table compartment, if the guy in front of you suddenly reclines his seat and it gets trapped you can snap your screen.