I can't choose instruments in preferences, there there's other stuff about midi output, looping, optimization for number of tracks or short delay, etc. However your suggestion, since the .mid file cannot be added to the left pane, made me to create a "new project". This opened a different window with just a "grand piano" track. When adding the .mid file there, it caused a crash and the program closed itself. On restart, creating a new project now showed plenty of instruments in a center pane, where I chose guitar. Now adding the .mid file was possible and it opened all the instruments into the .mid, but the sound is awful and I can't see any score to play along.
Thanks!