To provide you with a midi port that applications can use you need to launch timidity as a daemon.
Assuming that your sound driver uses ALSA, try opening an xterm (or whatever terminal emulator you prefer), and run:
It should show something similar to this and stay there:
$ timidity -iA
Requested buffer size 32768, fragment size 8192
ALSA pcm 'default' set buffer size 30104, period size 3760 bytes
TiMidity starting in ALSA server mode
Opening sequencer port: 128:0 128:1 128:2 128:3
Now you need to configure printmusic with the correct midi port, I can't tell you how to do that because I don't own a copy of printmusic, but looking around the menus you should be able to locate the correct place to set up all these details.
PS. If this works I can provide you with a sample script to automate all this stuff. I don't recommend running timidity as a global daemon by default all the time because it's a bit expensive in cpu usage.