Here's how to add ADE-DRMed .epub books that you purchased from stores other than Kobo to your Kobo Reader using Adobe Digtal Editions. You can probably also do this using the Kobo software but it won't install or run on my older Mac computers so I can't describe the procedure using the Kobo software:
1. Using your web browser or application of choice, purchase your books and download the .epub files. Make note of where they reside on your local harddisk.
2. Connect your Kobo Reader to your PC/Mac with the USB cable. Wait for the Reader to finish mounting (not strictly necessary for PCs but you must have the Kobo mounted on a Mac before starting ADE or it won't show up).
3. Start ADE. You should see the Kobo Reader listed at the bottom of the Bookshelves pane.
4. Add your recently purchased .epub file(s) to ADE by clicking the "Library" menu and selecting "Add Item to Library". In the resulting file browser window, navigate to the folder where you saved the .epub file(s), select it and click "Add". This will import the .epub file into the ADE library.
5. Open the book in ADE (just to be sure that you can actually access the book). Click on the Library icon (top left corner of the window) to return to the library view.
6. Copy the book to your Kobo Reader by dragging the cover of the book from the main window and dropping it onto the icon representing the Kobo Reader.
7. Unmount the Kobo Reader from the PC/Mac and unplug the USB cable. The Kobo should now display a message that it is processing new files. Shortly thereafter, your library books should be listed in your Books I'm Reading list with a "New" tag.