Whoops. I just read the info about the wiimote from the link above and found out that it uses a force sensor, not a true accelerometer. You can calibrate it for g's by just setting it down with the axis of interest vertical. Calibrate each axis separately in case they're different, and you might want to set it down, collect data, pick it up and move it around, then set it down with the same axis down and collect data again, just to eliminate possible errors.
The device should work just fine as an accelerometer in horizontal directions as is. If you want to measure vertical acceleration you'll need to subtract g from the measurement. Sorry for the confusion.