There is a good introduction to BIRT at http://www.eclipse.org/birt/phoenix/tutorial/—although the KNIME version is slightly different, it still offers information on few other options. Some of the views are not visible by default, so we will explain how you can create report designs for your workflow.
You might realize that when you installed the reporting extension, a new button appeared on the toolbar. The icon looks like four yellow/orange stripes and a line plot with four points. Also, it is on the right-hand side of the zooming factor. When you have saved your workflow, click it and apply the changes, so that the data from KNIME will be available as a data set.
Then, you open the KNIME report designer perspective, and you should get a dialog about the new data being available. It is recommended to apply the changes so that you will get the updated data in the designer, the preview, and in the reports.