I've followed the instructions here to install PostGIS http://trac.osgeo.org/postgis/wiki/UsersWikiPostGIS21UbuntuPGSQL93Apt but when I went to create the extension I got the following error. I'm using PostgreSQL 9.3.2 and Ubuntu 12.04. The extension doesn't appear in the dropdown list on pgadmin3 either.
# create extension postgis;
ERROR: could not open extension control file "/opt/PostgreSQL/9.3/share/postgresql/extension/postgis.control": No such file or directory