This is a story related achievement which will unlock with natural story progression.
To gain access to the butcher shop, you will need to locate the key as the door is locked. Locate Hardiman and he will tell you to find the two parts of the key. One half of the key is stuck in the outhouse door, and the other is upstairs outside his lodgings.
Once you give him the pieces he will meet you at the butchers shop where you can then enter and continue with the experiment. The achievement will unlock once you enter the shop.