You'll also need to install a virtual midi cable (which is just a piece of software that intercepts and re-directs midi messages inside the computer).
See this thread:
http://www.notation.com/vb-forum/showthread.php?t=3400
Then simply choose your virtual midi cable as the output of your virtual keyboard and set Composer to use that VMC as one of its midi inputs.
Setting up the VMPK is outlined on this page:
http://vmpk.sourceforge.net/#MIDI_Connections
Good luck.
David