Before using DbManager, you should load a shapefile you want to import to a database.

When ready, launch DbManager by going to DatabaseDB ManagerDB Manager. When the DbManager UI displays, locate your database, expand its node and select the schema you want to import the data to and then click the Import button (arrow down). You should be prompted with an import dialog; the Input dropdown lets you choose the layer to import.

Once you are happy with the import options (you may want to click the Update options button to populate the dialog), click on OK. When the import finishes you should see a confirmation dialogbox: