By default the Onboard software can record CDL Data from a Filmlight CDL Server (Pomfort LiveGrade uses the same protocol so this also works).
Set up a network connection from the Onboard recorder to either the laptop running the CDL Server software, or the Truelight box itself. If you need to change the network settings on the recorder this is done on the Setup->System menu screen. Note that after changing the network settings they are only updated when you press the 'Back' button.
In the Setup->System menu scroll to the bottom where there is the CDL Server setting. In here enter the IP address for the laptop or Truelight box. When using Pomfort LiveGrade you should also specify the port number after the IP address, e.g. 192.168.1.2:6666
Make a recording and afterwards go into Play->Shots and then open the Filecard for the recorded shot to confirm that the CDL values have been captured.