Are you using LabVIEW or C++? The specific answers are different for each environment, though the concepts are the same.
The "central voltage" is the voltage the accelerometer outputs when there is no acceleration. It translates to "zero G".
The "gain" is the voltage the accelerometer output changes by for a given measured acceleration.
The accelerometers we received in the 2009 Kit of Parts have a central voltage of 1.5 volts, with a gain of 300 millivolts per G. This information is in the
2009 Sensor Manual which you will find under
Section 8 - The Robot on the
2009 FIRST Robotics Competition Manual and Related Documents page.