I didn't knew anything about the mystuff
directory. Anyway I tried it.
mkdir /usr/ports/mystuff
mkdir /usr/ports/mystuff/x11
cp -r /usr/ports/x11/dmenu /usr/ports/mystuff/x11/dmenu
chown -R user:wsrc /usr/ports/mystuff
cd /usr/ports/mystuff/x11/dmenu
make install
And it was successfull, so you should check the
file permissions in and of mystuff dir.
If build was successfull in /usr/ports/x11/dmenu, then
permission could be skewed in mystuff.("local user":"wsrc")
If no conflict in that, I think you could have messed up
your /usr/ports/x11/dmenu dir before copying it,
did you edit it?