In order to preview, you will need to open the hierarchial view of your Modules/Sections/Topics.
When you open UPK, all of your content is shown on the right side - double click on a Module that you have created (you should have a section linked to this module, and you should have a topic linked to this section). Once you double click the module from the mail library, you will see that a tab opens with the name of your module.
Under the tab, you will see a window referred to as "name" on the left, and another window on the right referred to as "Concept/Introduction for". In the "name" window you should see your hierarchy for your topic. Drill down (click on the +) until you reach the topic you wish to preview.
Highlight (dont double click) the topic, and click on File --> Preview --> MODE.
Hope that helps!