After driving to the end of the road and passing the new pool in Seljavellir you have to walk across some gravel until you get to a stream which was more of a river when we were there due to heavy rain. At the river we met some Icelanders who were clever to bring rain boots to cross the river. They told us we had to cross the river and that the pool was worth getting cold feet in it. So we took of our socks and shoes and waded through the about 3 degrees celcius cold water. Behind the river/stream you have to walk further for about 500 metres and then you see the pool. There are even changing rooms there where you can hang your clothes.
The pool is not as hot as other natural hot pots in Iceland but it's still nice. Even in the rain and heavy wind that we had when we were there it's an amazing experience with a fantastic view of the mountains and little waterfalls around.