Hi Net,
All files on DCM are of two types each with two flavours.
They are either a .zip or .rar file. Zip files can be called .cbz and Rar files called cbr. This is so CDisplay can use them.
If you say you can extract the internal jpgs than you must have an archive app installed.
If you can see a file within Fett's uploads that ends with .sfv I think that is the problem file your Calibre app doesn't like. Click on it and hit delete. The rar file should then be able to be used inside Calibre.
Hope that helps,
-Yoc