Anyhow, start the app, open a psql shell with /Applications/Postgres.app/Contents/MacOS/bin/psql in a terminal window, and you get a session. Since (I think) the app is intended only for development use, I'm not sure if they intend for you to change those defaults. That's not to say that you shouldn't but just that the app is meant to be (nearly) zero conf. (The documentation suggests adding Applications/Postgres.app/Contents/MacOS/bin to $PATH, which is probably a good idea if you will use the command line tools often.)
So n other words, since they didn't bother telling us the user/password, this app is useless. Better off using MacPorts.
The /Application folder itself is owned by root:
achilles /$ ls -ld Applications/
drwxrwxr-x+ 55 root admin 1870 Jul 20 06:54 Applications/
But that wasn't what I meant. I meant /Applications/Postgres.app. That is owned by the user who installs it to /Applications. Here it is on my machine, for example: achilles /$ ls -ld Applications/Postgres.app/
drwxr-xr-x@ 3 achilles staff 102 Jul 18 20:17 Applications/Postgres.app/
Beyond that, you can login (with no password), this way: achilles /$ /Applications/Postgres.app/Contents/MacOS/bin/psql
psql (9.1.4)
Type "help" for help.
achilles=#
I'm sure it works because I just did it. The setup is arguably insecure, but again I don't think it's meant to be a production database. It's meant, as I understand it, for a single developer to use while coding.PS. and pardon me for being rude - it was late and was filled with too much cheap wine.