The devices connect via USB Cable to a PC or Mac. For Windows and Mac additional drivers are needed, so that the device is recognized as a Serial device. For Linux the drivers are included in the distribution. You can download the drivers for your OS from here:

• Windows – https://github.com/vlast3k/simple-co2-monitor-fw/blob/master/files/CH341SER.zip
• Mac – https://github.com/vlast3k/simple-co2-monitor-fw/raw/master/files/CH341SER_MAC-1.zip (consider also this link for installing on the latest version of the MAC OS:http://kiguino.moos.io/2014/12/31/how-to-use-arduino-nano-mini-pro-with-CH340G-on-mac-osx-yosemite.html)
• Linux – Drivers are included in the recent distributions. Just have a look on this post describing how to enable the COM Port
• macOS Sierra – https://github.com/adrianmihalko/ch340g-ch34g-ch34x-mac-os-x-driver