Your unique device ID and the firmware version you are running can both be accessed from the Info page of the main menu. Press "Menu" from the home page, then press your device Info icon (e.g. Samurai Blade Info or Shogun Flame info etc depending on the device you are using)
Your DID will be required to activate the Avid DNxHD / Avid DNxHR codec by obtaining a unique code from www.atomos.com/support.
You can check on the Atomos website to ensure you are running the latest firmware: http://www.atomos.com/support
Figure 1 - Info page (Blade, Ninja2 series)
Figure 2 - Info page (Shogun, Assassin, Flame and Inferno series)