The Pd 0.56-0 application compiled for Macintosh OS 10.10 or newer, available for download at https://msp.ucsd.edu/software.htm, is contained in an APFS-formatted .dmg disk image, and neither OS 10.10 nor OS 10.11 can mount APFS disk images. Perhaps some charitable soul will make the application available in an HFS-formatted disk image, the way previous versions of Pd were?
(Yes, I admit to being too lazy and clueless to compile it for myself.)
Thanks in advance!
Mike Green