You might want to contact DFrobot and ask them.
Depending on how the shield is setup, the sensors "ports" are probably connected to an analog pin. So you would be using "analogRead()" to get the value.
The light sensor should just be a voltage divider across 5v. So an analogRead() would be enough to give an idea how much light is there.
As for the CO2 sensor, you're going to need to find the datasheet for the sensor. You need to turn on the heating element for a certain amount of time, before you can take an accurate reading.