I am replying to a rather old post here, hoping to help out anybody who googled his way here in search of an answer (like myself).
I got the exact same message from mpc; in fact, almost every mpc command would return "you don't have permission for...", including the rather innocent mpc status.
$ mpc status
error: you don't have permission for "status"
In my case, the reason was quite logical: mpd was password-protected. Check your configuration files (/etc/mpd.conf and ~/.mpdconf); see if there is a line starting with the word 'password'.
$ grep '^password' /etc/mpd.conf ~/.mpdconf
grep: /home/ruud/.mpdconf: No such file or directory
Alright, so your password is 'foo'; specify it when calling mpc:
$ mpc --host foo@localhost update
Updating DB (#6) ...
volume: n/a repeat: off random: off single: off consume: off
HTH; and if so, you may want to set environment variable MPD_HOST, as proposed in the manual (man mpc).