Cannot send midi data to any application using Jack

I am probably being very dumb but am at the end of my tether.
I am trying to play a piece in Musescore and output to VMPK (Virtual Midi Piano Keyboard) . I dont seem to be able to connect the output of Musescore to anything using Jack but I can get VMPK to do what I want from other apps such as Virtual Keyboard - (press a key on one and the other shows the key as being pressed). Input from VMPK to Musescore also works. I have tried every conceivable connection arrangement in Jack to no avail. I know that it is possible as there are youtube vids showing the result so I must be missing something really obvious.
BTW , when I choose MIDI output and disable the internal synth and press the play button absolutely nothing happens - the cursor does not move over the notes or anything.
My system is UbuntuStudio 9.10 with all the latest updates installed
Musescore 0.9.4 revision 1505

Any help would be very gratefully received
Ian