a general COM+ interface needed when designing a custom FreeMASTER communication plug-in is available in the FreeMASTER 3.0 installation in "FreeMASTER\userdev\include\mcbcom_i.idl" file. Unfortunately we do not provide a source code examples or any detailed guide how to create the plug-ins. So far, there were no interest from users to do so.
I would recommend you to use one of supported CAN interfaces:
If it helps, we have also recently added a support for Viewtool Ginkgo USB-CAN interface. It is not officially released yet, but I can share a prototype with you.
We will consider adding the Lawicel device support by end of 2020.