If you want to install the GeoEngine on an existing database you have two options.
If you are using a PostgreSQL super user it should work out of the box. If you are using a standard user you have to connect to your database and run:
CREATE EXTENSION postgis
CREATE EXTENSION postgis_topology
In order to test if the installation is sucessfull log into you database and:
SELECT * from GEOMETRY_COLUMNS;
SELECT * from spatial_ref_sys;