About MIDI connection with my keyboard, it was just an error of
connection (I had to put the IN cable in the OUT port ...). I can now
play my MIDI files on my external keyboard, and save the notes from by
keyboard to Rosegarden. Super!
About synth, I have installed soundfonts and timidity. It's enough at
this time to play MIDI files, or get sound from Rosegarden.
Thanks for your support
--
Philippe