To download Adobe DRM books you need to install Adobe Digital Editions (ADE). This works pretty much the same whether you download from Kobo, Google, or one of the many other ePub retailers with Adobe DRM books. It is best to install a version of ADE earlier than version 3.0 if you want to be sure you can remove the DRM from future books once downloaded. I use version 1.7.2 and it works fine, but some others have found they need to use version 2.0.1 for Google.
You will also need ADE if you want to borrow pubic library books to read on your Kobo.
(An old version of ADE might not be so good for actually reading books with, but it is fine if you are just using it to download.)
Here is one of
Kobo's help pages on ADE. You don't need to actually authorize your ereader device if you don't intend to transfer DRM books to it, but you should create an Adobe ID and at least authorize the computer you have installed ADE on.
Here is the link for
ADE 1.7.2. (Windows)
Edit: You can currently still get ADE 2.0.1 from
this page.