SainSmart CO2 sensor module use?
The Dout is a TTL threshold indicator; basically it goes high when a certain threshold of the sensor is reached, which is adjusted by the pot.  The Aout is the VRef of the sensor.  

Now the MG-811 is not the best sensor in the world; I did look at the datasheet which shows this sensor outputs mV in relation to the PPM so a map function could be used such as 100-600mV maps to 400-10000ppm CO2, see MG-811 CO2 Sensor Module, so you could feed this into a map() function and get the result but since there is a signal conditioning circuit consisting of a op-amp I'm not sure what it does to the raw sensor voltage.

Now the sandbox electronics module may not be identical to the SainSmart module, so you may need to reverse engineer the module you have to determine what is going on or use the schematic from the sandbox site to build your own and de-solder the MG-811 from the SainSmart module.