(This information refers to the LEGO NXT robotics system. The information should be similar for LEGO EV3. However, this has not been tested.)
There is a way to connect a CO2 sensor to an NXT mindstorm, but it will require that you are comfortable with a breadboard. You also need to be aware that the output will not be calibrated or reported in ppm. However, you will see a value rise and fall (counts) that is related to the CO2 level.
You will need to make an external power supply for the CO2 sensor. You can purchase a plastic battery case that holds 4 AA batteries from many different electronic vendors. This will serve as your power supply. You will also need a BTA-ELV adapter, a BB-BTA, a breadboard, and some jumper wires. The BTA-ELV is an adapter that will accept the BTA connector from the CO2 sensor. Connect the BTA-ELV to a breadboard. You can get a standard breadboard from many different electronic vendors. Next, place the BB-BTA on the breadboard next to the BTA-ELV. The BB-BTA is an adapter that will take the inputs from the breadboard back to a BTA cable. Using a jumper wire, feed the 0-5V signal input from the BTA-ELV (SIG1) over to the same pin (SIG1) on the BB-BTA using the breadboard. Next, tie the Grounds from the BB-BTA and the BTA-ELV together using a jumper wire and the breadboard. Now connect the 5V and GND outputs from the battery case to the breadboard. Using a jumper wire, feed these signals to the appropriate lines of the BB-BTA.
To run Vernier sensors with LEGO MINDSTORMS you must install our Vernier Sensor Block. The Vernier Sensor Block download, and all of the installation instructions can be found at: http://www.vernier.com/nxt/downloads.html
Connect the BB-BTA to the NXT adapter. Connect the CO2 sensor to the BTA-ELV. Launch the NXT software.
After installing the Vernier Sensor Block, start a new project in the NXT Programming software. Drag and drop the Vernier Sensor Block into the programming environment. In the configuration window there is a control to select the Vernier Sensor. The CO2 sensor is not part of this list. Therefore, choose Raw Counts.
Below are links to the adapters that you will need
LEGO® MINDSTORMS® Education EV3 Core Set with Charger, order code LEGO-EV3-CORE, $412
NXT Sensor Adapter, order code BTA-NXT, $39
Vernier Engineering Projects with LEGO® MINDSTORMS® Education EV3 (Electronic Version), order code EP-EV3-E, $40
TIL 3339: LEGO® Mindstorms® EV3 Troubleshooting and FAQs