update: I was able to get it working by specifying the full path:
[declare -path /Library/Pd/<libfoldername>] and also [declare -stdpath /Library/Pd/<libfoldername>]

which seems odd, since the /Library/Pd is already in the standard search path, I would expect that it would automatically look there for sub folders pointed to with the [declare] method. anyway, it works!

oh, and in case anyone else is trying to do this, I believe [declare] currently only takes effect when you load the patch, so typing it in does not immediately add the library like the [import] object, you have to close and reopen the patch to see if your path was found.

curious about this, I'm having a hard time getting the library loaded in Pd-vanilla v0.44.0 using the declare method.

I have the library in my ~/Library/Pd directory and it shows up, and loads correctly when opening help patches from the browser, and also loads if I type in [libraryname/external ...] in an object, and using the [import] object in Pd-extended.

But I'm so far not able to get it to load using any of the declare flags. If the library folder is in your osx ~/Library/Pd folder, should [declare -stdpath libname] or [declare -stdlib libname] work for this location?

Looking at the pd window, I can see that pd is not looking in the library folder I declare when trying to find the external.