PostGIS "could not open extension control file" when creating extension
Looks like you have more than one PostgreSQL install. You're connected to a PostgreSQL install from EnterpriseDB, in /opt. This install cannot see, or use, extensions installed into the Ubuntu-packaged PostgreSQL install.

The instructions you followed will have installed PostgreSQL 9.3 from http://apt.postgresql.org/, i.e. PGDG. You should connect to that PostgreSQL install, not the EnterpriseDB one, if you want to use the PostGIS you installed. 

You need to either use the system-packaged (PGDG) PostgreSQL install on your system, or install PostGIS separately into the EnterpriseDB-packaged PostgreSQL install using their StackBuilder product.