To add an RJ Web Builder specific widget follow one of the articles on how to add widgets to a section or a page.
Here are the different RJ Web Builder widgets that are available to you:
RJ Web Builder RSS Feed (available feeds)
Looks like this:
RJ Web Builder Newsletter
One thing to remember when setting up this widget, you will need to create a JPG of your newsletter to use. Then upload that image to the media library and copy the link in order to paste in the Image URL field.
Looks like this:
The look and feel of each widget will be different depending on the theme you are on. The screen shots above were taken from Ariel.