I'm trying to add a doumbek to my score, but it's not listed as an available instrument under Instrument Change.  I'm using the Sibelius VDL 6.0 Template.  I have the doumbek sounds as a part of VDL 2.5, but I can't use it for an instrument change.  Any advice?

The doumbek in VDL was added a little later (my memory is fuzzy, but maybe library v2.5.2). So whichever template you're using would need to be up-to-date in supporting that library version. If the template you're using is older than that library version, you definitely won't see newer instruments in the Instrument Change dialog.