I'm showing step by step how to install PostgreSQL on Mac OSX.

install_postgresql_mac

# Uninstall all versions

http://stackoverflow.com/questions/8037729/completely-uninstall-postgresql-9-0-4-from-mac-osx-lion

# Download .dmg from http://postgresapp.com/
# Unzip the file and copy do Applications folder
# Run Postgres.app
# After this action will appear an elephant on menu bar: Postgres server is running
# Follow this steps from documentation: http://postgresapp.com/documentation#toc_1
# On command line execute

$ psql -d postgres

# That will open the postgre command line
# Will need create a new role to access the database. Execute

postgres=# create role USER_NAME login createdb;
postgres=# \q

# Where USER_NAME must be your username to access the database
# Try to connect on command line

psql -d USER_NAME