I had the same problem. The problem went away when I noticed that my Bible section was open to a OT passage. Since Barclay is a NT commentary, there was nothing for it to display. As soon as I went to Matthew, were were in business.
I do have a related question though. Barclay's NT Commentary is not listed in under Commentaries in the Downloads section. Is that to be expected, or is there something more that I need to do?