Currently, you can only change element attributes like its title, theme and state from the page configuration UI. The only action you can take from the Manage Includelets page is to delete the element (which perversely can't be done from the page configuration page, mostly to keep users from destroying their environment if they're allowed to have personal pagesets).
Below this list is a list of includelets which can be bound to elements and added to the pageset being configured. You can add an includelet as a new layout element as many times as you want. For example, you might want to include an instance of the forums includlet on each page displayed to the user. To do so, add the includelet as a new layout element once for each layout page that will be created for the subsite, then add them individually to each page in the page configuration UI.
When you're done adding includelets or deleting elements, click on the "return to add applications" button. This will allow you to manage includelets and layout elements for another application, or to resume adding new applications. If you're running the wizard, when you're finished click the "next" button to advance to the next configuration step.