bean, is the problem simply that foobar 2000 doesn't associate shn files with the program (i.e. that when looking at shn files in explorer you don't see a foobar icon associated with the shn files)?
If so, you might merely need to tell foobar to associate the shn files with foobar, which you do by hitting start>program files>foobar2000>foobar2000associations.
then, be sure the checkbox for shn is selected.
if the above doesn't work (and manually dragging a shn file into foobar won't cause it to play), you should confirm that the shorten decoder is, in fact installed, to do this, open foobar, select preferences from the foobar 2000 pulldown menu, select components (at the very top) and confirm that the shorten decoder is installed.