Connect to the database using a superuser account (most often the user named postgres but in more recent versions the id of the user who installed the application.) Then issue the following SQL commands to enable PostGIS functionality.
CREATE EXTENSION postgis;
CREATE EXTENSION postgis_topology;
You can do this from within pgAdmin or via psql -U [superuser] [database] from a command line.
Alternately for the command adverse; as a superuser; from within pgAdmin right click on your database’s Extensions and select New Extension. Then in the drop down associated with Name select the postgis* extensions needed.