How do I open a .djvu file?

Not sure where you searched, but a quick Google search found a lot of info- you can find three Mac compatible programs to open .djvu files here to get you started. Just Google "djvu" if you need more.

Well I don't really know anything about the .djvu files in general, but I downloaded the second viewer in the list (listed as DjVuLibre DjView) and then went out and downloaded a random .djvu file and the print function showed up fine as an icon in the program (see attached screenshot- the second icon) as well as in the Mac "File" menu as the standard "Print..." selection. Both ways of printing worked fine for me.

Glad that one worked ok for you.