To add the plugin to the panel, right click on the panel and select Add New Item. Select the plugin named Screenshot, and add it.
You can configure how screenshots are taken by right clicking on the plugin and selecting Properties. A preferences dialog similar to the dialog of the main application will appear. Please have a look above for the different preferences.
To take a screenshot, just click on the plugin. You can use the scroll wheel on the plugin button to switch between the different regions to be captured.