Downloading ebook to Adobe Digital Editions produces activation error

Hi there,

So I bought an ebook from the Google ebook store. I then wanted to change it to the Epub format, so I can read it on my Kobo Touch device. Using Digital Editions, I went to 'File', then 'Add to Library'. I went to 'Downloads' on my pc, and changed the file name to 'Adobe Content Server Message (*.acsm), which then showed the book file I wanted to download. But when I clicked on it, it came up with 'Error! Check Activation'. I see this happened to another user on this forum, but there's no answer to his question as yet. Please can someone help? So frustrated!

Thanks.