To install PostgreSQL and PostGIS on a Mac, you can use Postgres.app. You can download the file from http://postgresapp.com/. After the file has downloaded, move it to the applications folder and double-click it. Click Initialize. You will have a server on localhost:5432. The username and database name is the same as your Mac user. There is no password.

You should then be able to use the psql commands to create a new database and enable PostGIS.