installing Postgres

[Puneet Kishor]

This is a potentially silly question, but it would be sillier if I didn't ask.

I installed postgresql-90 server via mac ports, but am stuck unable to start the db. Mac ports creates an OS user called 'postgres' and I am supposed to

$sudo su postgres -c '/opt/local/lib/postgresql90/bin/pg_ctl start -D /path/to/db/postgresql90/defaultdb -l ~/Logs/postgresql90-server/main.log'

but, I don't know the password for the 'postgres' user, so I get the following error

pg_ctl: could not open PID file "/path/to/db/postgresql90/defaultdb/postmaster.pid": Permission denied

I have the db directory owned by postgres:postgres.

Suggestions?