In order to download a book bundle you should go to the book bundle download page, e.g. https://www.humblebundle.com/downloads?key=<your-book-bundle-key>. You can select the format(s) you want to download (pdf, mobi, epub). Note, it takes time for the "Save As" dialog to appear, because files need to be downloaded and then zipped. This is an experimental add-on and feedback is more than welcome.
