Coby tablets are shipped with an outdated version of Aldiko, which you will need to upgrade to the latest version (from the Android Market) before you can authorise it to your Adobe account and purchase books.
Follow these steps to install the latest version of Aldiko (free):
(1) Tap on this link on your device: http://aldiko.zendesk.com/entries/402881-download-the-latest-version-of-the-aldiko-book-reader-application. If you see a pop-up window saying unknown source, just tap "OK" to continue.
(2) The application should be downloaded automatically.
(3) When download completes, open the notification bar, tap on the apk file and then tap "Install".
After you have upgraded, be sure to authorise Aldiko using your Adobe ID, following the instructions here: https://aldiko.zendesk.com/entries/368907-authorize-device-with-an-adobe-id
You should then be able to download and read your book.