Well, there are several ways to install on OS X. But easy is great, and Enterprise DB is kind enough to maintain an installer. It'll prompt you for the installation location (including data), the administrative password, and the port number (defaults to 5432). The installer will even configure PostgreSQL to start automatically (although you can change that if you want).
And luckily, the PostgreSQL JDBC driver is in the Maven repositories (even the JBoss Maven 2 repository). That makes using JDBC a cinch:
<dependency>And of course the 8.3 manual (with comments). Now, we're all set.
Oh, and the installer also adds some nice OS X applications for starting, stopping, restarting, and administering databases. Just look under
/Applications/PostgreSQL 8.3(or the appropriate version).