Hi Franco,
I have not personally used Cantabile, but here are the general rules for using a host for your instruments via a virtual MIDI cable:
1. In Notation Musician/Composer, go to Setup/MIDI Devices Configuration/Port/Select Ports and make sure that the cable(s) are only selected as OUTPUT ports:
VirtualMIDIPorts.png
2. In your host (eg. Cantabile, SynthFont, whatever...) go to the similar setting for "Ports", and do the opposite. In other words, only have "In" or "Input" from the MIDI cable. Unselect the "out" or "output" for all virtual MIDI cables.
If you do not do this, then there is a possibility of setting up an "infinite loop" which can slow your computer to a crawl and you can't do anything.
LoopBe30 has a feature that detects such loops and shuts down the loop so it doesn't cripple your computer. I'm not sure about others.
3. For some hosts (eg. the Garritan Aria host) you will need to match the channel number in Musician/Composer's Staff Setup for each instrument with the channel number that is assigned in the host for the instrument you want to use:
ChannelMatching.png
Hope this gets you going!
Sherry