For players who aren't familiar with the game (but I assume you would be if you're buying the DLC :P)
Simply load up Mt.Moonspell, press start and you'll see a small question mark on your map, which is the location of the coffin.
There's an entrance towards the bottom right of the map, then just follow the dirt path when inside the castle.
Make sure to level up a decent amount before attempting, as you have to defeat all the enemies around the coffin first, before being able to open it, and the enemies around it are stronger than most.