Huh, this is interesting; I've never actually used the viewer version of the program, I just assumed it was the same as the designer one but you couldn't save... ok. What you'll need to do is download the designer, not the viewer (you can get a free trial here, which will have all the functionality of the full program, just not the ability to save: [link] ).

When you first open the program, you'll notice that the screen Is divided in half vertically. The left will show the 3D representation of the model, while the right will show the pieces 'unfolded'.

Basic movement
For the 3d view:
> click-and-drag with the right button will rotate the view
> scrolling with the mouse wheel (if you have one), or shift and click-and-drag with the right button will zoom in and out
> click-and-drag with the mouse wheel (if you have one), or Ctrl and click-and-drag with the right button will pan around

For the 'unfolded' view:
> scrolling with the mouse wheel (if you have one), or shift and click-and-drag with the right button will zoom in and out
> click-and-drag with the mouse wheel (if you have one), or click-and-drag with the right button will pan around

Now, to use this program to check edges: You need to select the "Divide/Connect Faces" tool. You can either hunt out the little button in the bottom horizontal bar at the top (it's a blue box with a silver horizontal zipper running along it), or press Ctrl+N.
To use it, move the mouse over any pieces edge (in the right 'unfolded' window), and it will highlight both connecting edges in red andh draw a line between them to lead you from one to the other.
Warning: Do NOT actually click the edges, because since you're using the designer, it will actually edit the model, and we don't want that

That should just about do it. Feel free to experiment with the other tools in the program, but the only other one I think will really help is the "Check Corresponding Faces" tool. With this, you can click any face in the 3D left window, and it will highlight the face in the right 'unfolded' window, and vice versa. The button is a few to the right of the divide/connect tool, and the icon is a cube with a question mark on it (or just press Ctrl+K).


Assembled Image
Instructions
PDF of Pages 1-10


If you can't figure out how to download this file, referr to this One-Step How-To Guide