Go to the Project View Properties menu, and to the Legend tab. Make sure the Show Legend box is checked.
In some cases, you may be showing the legend, but have inadvertently moved it off the screen. This can sometimes happen if you've resized your project view to be smaller, and the legend had no place to go. It's pretty easy to get it back, though.
On the same Legend tab, in the upper-right-hand corner, you'll see a set of radio buttons that say Location on Screen. Pick any one of these six positions and OnePager Pro will reposition your legend into that spot on the screen.
Once your legend has been repositioned, you can still drag it around and fine-tune its positoning.