EnterpriseDB provides a wonderful one-click installer for PostgreSQL on OS X that comes packaged with several contrib modules.
If you need to enable any of these, you can find the SQL in /Library/PostgreSQL/8.3/share/postgresql/contrib/. For example…
$ psql -U postgres template1 < /Library/PostgreSQL/8.3/share/postgresql/contrib/fuzzystrmatch.sql
Remember to execute the SQL on any pre-existing databases in which you want to use the specified contrib functions.