Ladies and gentleman, I feel dumb but if you look above the code on your receipt (Maybe not people from the midnight launch but mine anyways states) **PLEASENOTE** The metal Gear Rising Codes are currently not live please keep this receipt!
Mine doesn't say that and the items are available on PS3 already.
That's because they're only slow on making Day 1 DLC available on XBox Live. It's not the first time I've run into this. Another way PS3 owners have the advantage over XBox 360 owners.
"It takes a real man to fight with a giant pink scythe." - danstu
At least they were able to give the codes out even if you can't use them yet. It's better than the issue they had with Halo 4 where they couldn't even print the codes and people had to go back to the store the next day just to get them.
Codes will work sooner then later. Last time this happened was with NBA 2K13, the codes began to work in the afternoon hours of the release date. Just be patient and do not throw out your code, it will work later on.