The labels file should have been installed when FileMaker was installed. The exact file varies with the locality specified for the installation.
On my Windows 7 system, the file path is: FileMaker 13 Advanced | Extensions | Labels
I would expect a similar set up on your Mac system.
Was just coming back here to post the solution; thanks, Phil.
I'm not sure why the v12 upgrade did not keep the Labels file, or why it eradicated it, but a friend on another forum just pointed me to this:That was the solution -- I found the file noted (LabelsUS.flb) buried in a previous version (v11, which I zipped and stored on an off-drive backup). I set up a new "Labels" folder in my v12 extensions, moved the LabelsUS.flb to it, and relaunched, and there were all the Avery templates.A little bit of tweaking and the one I needed (5163) printed without issue. Saved me a ton of time -- it was a simple fix once I knew what i was looking for.