I assume you have got the .acsm file ok, but can't use it to download the .epub?
Two horrible suggestions ....
Looking at https://getsatisfaction.com/kobo/topics/having_issues_downloading_an_a de_epub_from_the_kobo_site
I suggest editing the .acsm file you have.
If it has got a line something like <downloadType>simple</downloadType> but with a different type mentioned, edit it so the type is simple.
If it doesn't have such a line, add it just before the </resourceItemInfo>
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Someone else said they managed to read a book with Sony Reader that ADE would not manage.
Try at http://www.sony.co.uk/support/en/topics/reader-for-pc
If you don't have a SOny you can probably still install the software and use it on the PC.
Make sure it is registered with same Adobe ID as ADE (and your Kobo).
If you do manage to open a book that way, you can then 'sideload' the .epub file from PC to Kobo by copying using file manager such as Windows Explorer.