Problem installing PostgreSQL 9.0 via Macports on OS X Server 10.6

[…]
---> Installing postgresql90-server @9.0.5_0

To create a database instance, after install do
sudo mkdir -p /opt/local/var/db/postgresql90/defaultdb
sudo chown postgres:postgres /opt/local/var/db/postgresql90/defaultdb
sudo su postgres -c '/opt/local/lib/postgresql90/bin/initdb -D /opt/local/var/db/postgresql90/defaultdb'

To tweak your DBMS, consider increasing kern.sysv.shmmax by adding an increased kern.sysv.shmmax .. to /etc/sysctl.conf
---> Activating postgresql90-server @9.0.5_0
---> Cleaning postgresql90-server
Sun Oct 23 12:45:49 -- server :: sudo mkdir -p /opt/local/var/db/postgresql90/defaultdb
Sun Oct 23 12:48:41 -- server :: sudo chown postgres:postgres /opt/local/var/db/postgresql90/defaultdb
chown: postgres: Invalid argument
Sun Oct 23 12:48:51 -- server ::

So, according to the above instructions (and the tutorials I've read), this should simply work. What am I missing?

On a different note, I have a couple questions concerning PostgreSQL security:

1. My understanding is that making the defaultdb directory owned by the postgres user is for security. Correct?
2. Whenever starting psql, I have on my own machines had to specify "psql -U postgres", because:

Sun Oct 23 14:27:28 -- Frodo :: psql
psql: FATAL: role "rene" does not exist

So, I do I need to add a role/user for my OS X user name?

Coming from MySQL, I find the devil is in the details. Just trying to figure things out… Thanks!

…Rene