The lake Frostastaðvatn is located close to Landmannalaugar. You are going to pass it when driving into Landmannalaugar no matter which road you are coming from.

Very short before driving to the campsite you are passing the lake on your right side while driving up a small hill. It is a beautiful lake with an island in the middle of it. On the side facing the mountains you can see an old lava stream coming down. It stopped in a way that it still looks like flowing into the water even though the outbreak is hundreds of years ago.

I recommend to use one of the small parking lots, either half way up the hill or on top of the hill and get out of your car to enjoy the scenery and take some pictures.