The cache is beside a path, it's quite a steep but short walk from the bridge. If you're driving, you can park along the road and follow the footpath down to the river. Make sure you wear some decent shoes if it's icy or slippery and it can get quite windy here.
Hveragerði is the green house town and has plenty of places to walk. While you're there, take a look at the shopping mall for some interesting exhibits and make a stop at the bakery!