Normally we never need access to the actual files that make up the database. We have command line access via the 'psql' tool. We have GUI admin apps such as the bundled 'pgAdmin', and from countless 3rd-party vendors. We have the 'pgDump' and 'pgRestore' tools to backup and restore the database's contents. Because of all those tools, you may never need to actually pierce the veil to peer at the actual disk files that make up the database.
But just in case you ever do have such a need, or curiosity gets the better of you, you can use the Terminal.app this way:
sudo su postgresType that into Terminal, hit return, and use your regular Mac admin account password as prompted. The word 'postgres' is the name of the special user created by the Postgres installer. Be very careful – you will be working with all the "safeties" turned off, enabling you to take actions that are normally forbidden to protect you from yourself.
To leave that session, type: