To connect to the database, right click on its node and select "Connect" from the popup menu. After you do this the wiggly white line disappears from the icon for the database changes as shown below, indicating that it is connected. Then you can expand the node to see what's there. The "APP" subnode is where NetBeans will put database tables.
Most of the nodes shown below are best left alone. The "APP" node is where you will be adding database tables.
First, open the "Databases" node and then the "Drivers" subnode in the NetBeans "Services" pane. Then right-click on the "JavaDB (Embedded)" node and select "Connect Using.." from the popup menu. This brings up a wizard with three steps:
When you complete the wizard a new connection will appear.
The default schema should be used.
You can change the name to a shorter name if you prefer or accept the default name.
A new connection should appear below the "Drivers" node.