"Webmail is turned off." But it's not

Question

First off, I have searched the forums (and Google) but nothing that's turned up has been relevant to my set-up.
I have multiple Drupal 7 web sites up on the same Mac Mini Server, and they all work just fine, even though all of their files are stored on an external drive connected to the Mini.  I say that to establish that I've already found and implemented the fixes to get virtual hosting working.
I have the Mail Server up and running as well, and can successfully send and receive mail via IMAP accounts both while logged into the Mini and also remotely by pointing the Mail app on another Mac to the Mini and setting up an IMAP account there. Everything works just great.
The issue is webmail.  When I point my browser to www.example.com/webmail I get this:
Webmail is turned off. You can turn it on by using the Server app on the server.
Clearly it is turned on, else I wouldn't be here with my puzzled hat on.
My router is a Time Capsule. I have a lot of ports forwarded to the Mini but there's no entry there for a "Webmail" service--but this should just go in as HTTP, right? I don't believe any port other than 80 is applicable here.
For the Web Server configuration for the site in question that I want Webmail working on, I have www.example.com and example.com set to point at the external drive volume where that particular site's Drupal files live.  Does the Webmail access freak out because that domain's web site isn't at some default location it expects to see when looking for the Webmail interface files?  If so, where are they and how do I get to them?
Many thanks in advance!

Answer 1

So, I've done some more digging. Thank you Apple, for thoughtfully providing documentation on your included software!  All you told me was that to enable webmail I had to check one box in the Server App.
Sigh.
Okay, rant off.  It's pretty obvious from the log entries I posted that both a user and a database are missing from postgres.  After some searching I found an INSTALL document, that says this:
* PostgreSQL
To use Roundcube with PostgreSQL support you have to follow these
simple steps, which have to be done as the postgres system user (or
which ever is the database superuser):
$ createuser roundcube
$ createdb -O roundcube -E UNICODE roundcubemail
$ psql roundcubemail
roundcubemail =# ALTER USER roundcube WITH PASSWORD 'the_new_password';
roundcubemail =# \c - roundcube
roundcubemail => \i SQL/postgres.initial.sql
All this has been tested with PostgreSQL 8.x and 7.4.x. Older
versions don't have a -O option for the createdb, so if you are
using that version you'll have to change ownership of the DB later.
Okay, good...except it doesn't tell me how to get to the $ prompt for entering the first set of commands.  On a hunch I tried just typing "postgres" at the command prompt and got this:
postgres does not know where to find the server configuration file.
You must specify the --config-file or -D invocation option or set the PGDATA environment variable.
I Googled that whole section as my search item and about the only relevant thing I got was info on how to set one's environment variables--which I already know how to do. What I don't know is where this variable is supposed to point.
Okay, I'm at my limit, I've run out of fingers to hold on various bullet points. Is there some clear, concise how-to on setting all this up that doesn't assume I already know half this stuff?