CREATE EXTENSION adminpack;
CREATE EXTENSION postgis;
This installs postgis in the current database, with all data types and operators and the adminpack
extension that it needs (adminpack is already on your system, most likely). Note that you have to do this for every database that you create, unless you create the extension in one of the template databases (not such a good idea anymore since 9.1).
Note that there is a difference between installing an extension on your file system and making it available in your database.