Thanks! I finally got back to this and found a very clear tutorial about paths and dynamic libraries. In my case, I think the externals are loaded because Pd constructs an explicit full path to them, but they depend on a shared library that I thought would simply load from the same place or maybe the executable directory. But no, a "full path" is needed. I put that in quotes, because, while the error message says you cannot use a relative path, the solution in fact is to use a relative path! The trick is that you have to provide an explicit relative path policy (this is not Apple's terminology and they seem a little confused). Policies include executable-relative, loader-relative, and RPATH-relative. In my case, I needed to write @loader_path/libo2pd.dylib to mean "find libo2pd.dylib relative to the path where the loader found the Pd external." I made things work using the following:

install_name_tool -id @loader_path/libo2pd.dylib libo2pd.dylib
install_name_tool -change libo2pd.dylib @loader_path/libo2pd.dylib o2ensemble.pd_darwin
install_name_tool -change libo2pd.dylib @loader_path/libo2pd.dylib o2property.pd_darwin
install_name_tool -change libo2pd.dylib @loader_path/libo2pd.dylib o2receive.pd_darwin
install_name_tool -change libo2pd.dylib @loader_path/libo2pd.dylib o2send.pd_darwin

where the .dylib file is the one loaded and required by the .pd_darwin files. There must be ways to specify all this as linker options, but I haven't gotten there yet.

I hope this might help someone make sense of their dll problems.

I'm running Pd-0.52-2 on MacOS Monterey (12.6) and having trouble with externals that I created. When developing/debugging, everything is fine. I have 4 externals with extensions .pd_darwin, each dependent upon a DLL named libo2pd.dylib. With everything in ~/Documents/Pd/externals/, I can start my locally compiled pd-0.52-2.app, and everything works. But if I run a downloaded /Applications/Pd-052-2.app and try to create one of these external objects, I get the message:

/Users/rbd/Documents/Pd/externals/o2ensemble.pd_darwin: dlopen(/Users/rbd/Documents/Pd/externals/o2ensemble.pd_darwin, 0x000A): Library not loaded: 'libo2pd.dylib'
  Referenced from: '/Users/rbd/Documents/Pd/externals/o2ensemble.pd_darwin'
  Reason: tried: 'libo2pd.dylib' (relative path not allowed in hardened program), '/usr/lib/libo2pd.dylib' (no such file)
 o2ensemble o2global
... couldn't create

I get the same results running from the command line, and running from the directory with the .dylib and externals does not make any difference. Note that /usr/lib is protected even using sudo, so even if I could install libo2pd.dylib there, I don't think it's an acceptable solution.

I suspect this problem stems from Apple security policies, but I don't know how Pd searches for .dylibs and there's a lot of ambiguity in documentation and discussions since "libraries" and "paths" are used in multiple ways. I also tried setting LD_LIBRARY_PATH and running from the command line, but that didn't work.

Does anyone know to set up Pd for an external that depends on a DLL?

Thanks! I'm been traveling and just had a chance to try again. I erased all the pd stuff I had downloaded before and started from scratch. I used http://msp.ucsd.edu/software.html (pd-0.52-2.src.tar.gz) and your commands: ./autogen.sh; ./configure CFLAGS=""; make CFLAGS="" followed by cd mac; ./osx-app.sh 0.52-2; open Pd-0.52-2.app;
It worked! Possibly an earlier update/reinstall of Jack was needed, and I'm not really sure what Jack-related stuff Pd is using now (if any).
I think my earlier problem was thinking "Pd-extended community version" was a better place to start with development and writing an external, and possibly starting fresh cleared some shell variables set in earlier failed attempts.

I'd like to follow the steps you actually use. Where do you get Pd source code?
One of the problems I ran into was make not finding Jackmp. How did you install Jack?

Thanks for your reply. My question was whether there are any instructions that are tested on macOS 12? That's still a real question. To answer "what about it didn't work", we'd have to identify "it", but there are many sets of instructions. I tried several. I could pick one at random and provide details (the farthest I got was compiling a Pd that seemed to be running but the display was almost completely black), but for starters, what instructions do you recommend? Thanks.

I found lots of instructions on compiling Pd from sources for macOS, but after trying 3, and none actually worked, could someone point me to build-from-source instructions tested on macOS 12 for any Pd version?