The navigation buttons will appear down the bottom of the screen when you are viewing a lesson.
Note that most pages will have buttons to view additional content. The "Next" button will be disabled untill all content is viewed.
On some pages blocks of content will be contained in dropdown buttons, they can be identified by the arrow pointing down on the right side of the text. To see the content you can click anywhere on the title.
Some pages will have buttons that open a popup window with extra information. After reading the popup it can be closed by tapping the "X" in the top right or the "Close" button at the bottom.
Some lessons are layed out in number of slides. You can navigate forward with with the "Next >" button and backwards with the "<" (Back) buttons.
You can see how many slides there are and where you are up to using the small dots below the navigation.