From the fast travel point in Flamerock Refuge, turn right and head across the bridge then bear left through the archway and down the steps into the open area at the southern-most point of the map. You'll probably be attacked by a few spiders at this point so kill them, then head up the long flight of steps which were facing you when you entered this area. The Lootapult is at the top of these stairs and it costs 10 Eridium to fire it. The achievement should pop immediately the first time you use it. Don't forget to go back downstairs to collect the contents of the chest