Now you can create a test database with /usr/local/bin/createdb test
Use it with /usr/local/bin/psql test
If you would like PostgreSQL to start up automatically when the system comes up, then you can install this package which installs a StartupItem into /Library/StartupItems: pgsql-startupitem-1.2.pkg.tar.gz
/Library/StartupItems/PostgreSQL/PostgreSQL
Here is a little image that you assign to the new postgres user account in the "Users" system control panel so it looks a bit nicer in the login dialog: postgres.tif