Okay this achievement is pretty straight forward as it says unlock a treasure chest for 5 Gamerscore.. After the first level you'll be able to finally unlock a chest, the first one you come across is right after the easy boss ok level two after you beat him follow your path it'll tell you to use the staff to talk to the Golden statue for guidance and then it shall point you to the first chest which is 100 Gems simple.
Another easier shorter method for this is.... Just press
in front of any chest when you have enough gems.
Good Luck, not that you'll need it for this one.