In the Servers treeview, expand the registered server. This will show the Databases treenode:
An expanded NexusDB server
Database Workbench will list the databases available so you can connect to them.
Double clicking a database will connect and open a Database Navigator window. Alternatively, right click on a database and select Connect As to connect including a username/password dialog.
Options
Right clicking a database and selecting options will display the following dialog:
Database Options dialog
The General and Log Files tabs are handled in Database Options.
In the above dialog, you can specify a username/password to be used for this database only. You can also specify multiple "table passwords" if your tables are encrypted. These passwords will be set for the session after you've connected to your database.