Thank you for the speedy reply!
I'm still missing a critical step somewhere b/c ExifTool isn't seeing the DirTree.config file. It now exists everywhere (e.g. my home directory, the root directory with the ExifTool Unix Executable, /usr/local/bin/lib, /usr/local/bin/lib/Image, /usr/local/bin/lib/Image/ExifTool/) and I still can't get it to work. I read in other forum posts that I could specify the config file location, but that caused more problems/errors (not sure why).
I went back through the example.config instructions and I've followed them to the letter (and then did it again just to make sure). I've also reinstalled ExifTool and then repeated the steps above. I'm sure it's something simple that I'm missing, but it's eluding me.