For my Intel-based Macbook the directions are as follows:
Note: Portable computers that have a battery you should not remove on your own include MacBook Pro (Early 2009) and later, all models of MacBook Air, and MacBook (Late 2009).
Shut down the computer.
1. Plug in the MagSafe power adapter to a power source, connecting it to the Mac if its not already connected.
2. On the built-in keyboard, press the (left side) Shift-Control-Option keys and the power button at the same time.
3. Release all the keys and the power button at the same time.
4. Press the power button to turn on the computer.
Note: The LED on the MagSafe power adapter may change states or temporarily turn off when you reset the SMC.
If you have a different MacBook, more information can be found here: http://support.apple.com/kb/ht3964
If resetting the SMC does not work, you may want to try resetting the PRAM. Below are the directions:
- Shut down your Mac.
- Locate the following keys on the keyboard: Command (⌘), Option, P, and R. You will need to hold these keys down simultaneously in step 4.
- Turn on the computer.
- Press and hold the Command-Option-P-R keys before the gray screen appears.
- Hold the keys down until the computer restarts and you hear the startup sound for the second time.
- Release the keys.