This is a great tips for every MacBook Air users, directly from MacTips (my favorite mac tips website) .
So you just bought the wonderfully tiny MacBook Air and now it won’t turn on, or maybe it doesn’t want to charge the non-replaceable battery. Either way, you probably need to reset the PMU. How do you reset the PMU on your MacBook Air?
In order to reset the PMU of your MacBook Air follow these steps.
1. Turn off MacBook Air.
2. Connect the power adapter and plug it in.
3. On the LEFT side of the keyboard (Yes, the left side only), hold down Shift, Option and Control.
4. Press the power button.
5. Wait 5 seconds, then release all keys.
6. Push power button to turn your MacBook Air back on.
Caution, make sure the MacBook Air is completely shut down before reseting the PMU to avoid damage to your file system.