Seljalandsfoss is located between Selfoss and Skógafoss at the road crossing of Route 1 (the Ring Road) with the track going into Þórsmörk. It is about 30 km from Skógar. It was seen on The Amazing Race 6, when it was a waypoint during the first leg.
You can view what it's like to go behind the waterfall here: http://www.youtube.com/
Image: alba-7 on http://www.flickr.com/