In order to use PACX with Microsoft Flight Simulator (MSFS 2020), FSUIPC 7 is required. FSUIPC 7 is a utility that allows applications to connect to and interact with various simulator platforms. It is not developed, maintained, or sold by TFDi Design.
As of now, FSUIPC 7 is available as a free beta from here. Once downloaded, extract the contents of the zip file to a memorable location. When you ready to fly, open your simulator, then open FSUIPC7.exe. This will allow PACX to communicate properly with your simulator.
Please note: this article applies only to Microsoft Flight Simulator (released in 2020), not to Flight Simulator 2004 or Flight Simulator X. PACX automatically installs the required dependencies for other platforms upon first run.