Zim does not have printing support. There is a work-around to "print" to a browser, so you can use the browser to do the real printing. This is implemented in the "PrintToBrowser" plugin.
In order to do printing natively we need to use the Gtk printing interface (introduced in Gtk+ 2.10). This involves rendering the contents of the page to a cairo context. Examples can be found on the internet how to do this for text only using Pango. We need a little bit more advanced implementation that also takes care of images.
See also: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=343369