Hi Stefan,
I will first recommend you install latest FreeMaster 3.2 from https://www.nxp.com/design/software/development-software/freemaster-run-time-debugging-tool:FREEMAST...
Then open the *.pmp project in FreeMaster 3.2 and then go to Project -> Options:

Here select your appropriate COM port and speed usually is 115200.
You should see COM port number for OpenSDA - CDC Serial in Windows Device manager:

After all this is sorted, click on the FreeMaster -> File -> Start Communication.
PS: before doing all of this, you need to open the model in Matlab and build it. this will create the needed variables for FreeMaster.