We are continuing to look into the issues people are experiencing with The Pitt for Xbox 360. As of now it looks like the file for the English version on Xbox Live was somehow corrupted. We are currently working with Microsoft to remove the existing file off of Live to prevent any further downloads. Our plan is to replace it with a new file as soon as possible. We hope to have another update for you shortly.
It sounds like this is only impacting Xbox 360 owners, so if you're looking for the PC version of the add-on, you should still be in business.