Maybe you could try writing to the tutor contact mentioned in the booklets? They might be able to help you. Maybe there has been a change since I made my downloads; maybe you are only allowed to download one set of booklets at a time, then maybe the next one is unlocked? Have you worked with the first set of booklets and sent someone, maybe your tutor, some kind of feedback to show you’ve been doing the study? It may be that the studies have been divided into chapters which must be completed one by one.