There are two ways to see Voringsfossen...from top and from the bottom. If you choose the first one, be prepared to be with the crowd since Voringsfossen is a very (if not the most) popular waterfall of norway. A short trail leads you to some nice views of the falls from top. There is also a visitor center / restaurant where you can get information and directions to the trailhead to reach the bottom of Voringsfossen.
This is option two which I would prefer...less people, better views. The trailhead is at a small roadside parking lot (to the right) just before the tunnel, when you drive back from the visitor center to Eidfjord. Except for the first part, the hike is rather flat but still not too easy since you have to hike through stone fields with huge boulders. Roughly one hour should do it to the falls (depending on your walking speed).