I'm sorry but I don't have practice with iMac.
But the error
means that there is no such user (in /etc/passwd or elsewhere).
It seems that postgresql is not installed correct.
Pleas try to uninstall and install again (after reboot?).
Why apache should starts postgresql?
There is no reason for the apache server to that because apache has nothing to do with postgresql.
Or did you installed the XAMP or XAPP bundle or so?
If yes, pleas refer to the XAMP documentation concerning iMAC / FreeSBD.
Otherwise you can install the independed software apache (and php) and postgresql.