In order to add content to your page, you must create an Assets. Assets could be thought of as small boxes that contain the elements you wish to present. Once you have created your page, and defined your page properties, the next step is to add an Assets.
For example, should you decide to add a block of text, you can use the text asset. There are many different assets to make adding content easy.
In order to add an asset, you can either click on the "Content" link found in your "Edit Site Pages" area of SB3:
or on your "page properties" page, click on the "Add new asset" link found on the top of the page: