After putting deken-plugin.tcl into extra/, don't you see a message from Deken in the Pd-console when you start Pd?
no, I don't
I must assume you downloaded deken-plugin.tcl by copy&pasting from browser to Microsoft Word and saved it as deken-plugin.tcl.
nope, I downloaded as it is.
So, if I just put in "extra", I get no message, no change in behaviour, and, like I said, my pd has a pd_deken.tcl file in a tcl folder outside the extra folder, that's the one that needs to be replaced, so it seems...
So, I just repeated myself, so what does it mean? I'm trying to say that this file that is there still counts... how do I know? If I delete it, I get a tcl error saying the file couldn't be found... even if I have that other one in the extra folder, like you suggested.
Now, if I replace the tcl file I have for the new renamed downloaded file, like I also said, nothing new happens, it doesn't offer me to install in the non existing ~Library/Pd folder.