To help future people... despite the OSX commandline stuff all working fine, this plugin wouldn't correctly run the commands. I'd get errors when I clicked on the button to create the columns. Running calibre via commandline, I did get the commands it was trying to run, and running them manually worked fine:
calibredb add_custom_column --library-path="/path/to/library" ddc DDC text
calibredb add_custom_column --library-path="/path/to/library" lcc LCC text
calibredb add_custom_column --library-path="/path/to/library" --is-multiple lcead "LC Extra Author Details" text
calibredb add_custom_column --library-path="/path/to/library" oclc_owi OCLC_OWI text
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