Hi
Pietari,
Glad to hear of your interest in working with FluidSynth and ChucK. The main FluidSynth chugin works fairly well on Mac OS X and Linux (last I checked) but I have not yet tried to port it to Windows. Heavyweight chugins like FluidSynth tend to be harder to develop and maintain for multiple platforms, as each system has its own eccentricities with build systems and linking to external libraries.
If you can post the log output from the console when that error occurs, that would be helpful, as maybe it is a straightforward error to fix.
spencer