You can easily access the geometry database by clicking on theicon in the toolbar of CASD. Alternatively, you can access the database from the Geometry menu.
When you open the database for the first time it will be completely empty so you either need toto an existing database or to a new database:
You will normally connect to a geometry database previously created in CASD or imported from a different CAD software. The import procedure is explained in the article How to import a geometry. The other option is to create a new database and build the geometric model manually, as discussed in Working with a geometry database.