MacSQL uses a slightly different terminology for database objects than Sybase.
What MacSQL refers to as a database is called the owner in Sybase. There is no way to represent a Sybase database in MacSQL after a connection is open.
However, you can specify the initial database that you would like to work with in the New Connection window. If you do not specify one (or specify a database that does not exist or that you do not have permission to access), you will instead work with the default database assigned to you by your database administrator.