Okay, so this for anyone who is planning to go here: this place is very difficult to locate! At least, if you're used to American/Western layouts of stores. I'm used to stores being in a mall, on a street, etc. Typical stuff, right? Wrong!
This place is visible, very visible, from both the Macro Art Museum near it, and another corner, but to get to it you have to do some serious walking. Not necessarily far, just...serious.
We departed from the Piramide metro station, and from there you walk towards the Piramide, against traffic, towards Via Caio Cestio. You'll take a left on Caio Cestio and walk towards Via Nicola Zabagalia. Cross that street and go down the little path in front of you (this area reminded me of a college campus; there are pillars there to prevent cars from going down as well). Now you are on Via di Monte Tessacio. This is a street that does a big "U". Following the directions here you'll want to head left after going down the little path.
Walk all the way around the U until you come out to a big parking lot and see a giant wall, sort of castle-looking thing in front of you. There's an actual parking lot (a big "P") on your left, a restaurant on the right and you'll want to walk towards this wall. Once you reach the wall, turn left and walk along it. The shop is basically on the other side, but it's not that easy...
You'll pass another club on the right and then find an entrance, basically a gap in this castle/wall thing. Go straight - we did this at night and it was really creepy, so if you're going at night and it looks creepy, you're in the right place! On the left was some type of weird gathering, probably another restaurant or something, but stay straight on this path. If you look right and can see through the fence you may be able to see the store as it's well lit.
Continue down this path until you find an opening in the fence on your right. The opening should give way to a large field; I assume this was part of the slaughterhouse referenced in the description. You need to cross this field in order to get to the entrance of the store.
How is the place? It took us so long with so much trial-and-error that by the time we got there...it was closed :(. Also, HC needs to be updated to reflect that it closes at 20:00, not 20:30. Alas...if you go there, please let us know how it is!