Still i dont know how to make it works...
Well, the easiest thing would be to copy the external *.pd_darwin file into your /path/to/pd/etra folder, which is always included in the path. Otherwise, in your home directory's .pdrc file, add the line -path /path/to/the/external for whatever external you want.
I dont know where is that folder, ive got a compilation that somebody gave to me, when i open pd the wish window and terminal are opened. I did copy the Pd aplication in my Aplications folder, and i made(i think is silly) a new folder in the same place called Pdexternals. But i cant find another folder with that name...