Mosfellsbær is on the outskirts of Reykjavík and is most well-known for it's connection with the wool industry and it's links to the Icelandic Writer, Halldór Laxness, a nobel prize winner. If you are without a car in iceland, this cache is accesible via the number 15 bus, where you can get off at the Hotel Laxnes' stop, there is also a carpark near by.
Make sure you bring a pen to sign the log because there's no room for anything to write with in the cannister!