Think you guys are being too harsh to be honest. We received the original email reply from them, which was asking for the backer code & version number at around mid-day Friday UK time. Already, that's like 9PM on a Friday in Japan where they are located. Of course they aren't going to reply for a few days, they are on their weekend and not even in the office. They have replied today, which is the next working day so it's not 4 days, it's 1 working day (if you can even call it that because they would have been on their weekend when we sent over the original replies anyway).
Either way, it's also clear that they don't think that it's user error, as 1) they open the reply acknowledging that there is a problem & 2) they are asking for information trying to work out why it's broken. The only reason they are asking about where/when we entered the Backer Password, is because they are trying to work out if the issue has something to do with where it was entered. It might be broker if you enter from the main menu, but might work it you originally entered it via the In-game menu. If they receive 50 emails where everyone entered on the main menu and none from the in-game menu, then they know where the problem lies.
They have a lot of people emailing them about this, they are aware of the issue and they will fix it. Yes, I know that it's annoying having to wait but have some fun and play the game, the DLC will work soon.