If you've downloaded your eBook prior to having the proper e-reader software, your computer or device will have no application to open it with. If you are on a PC, simply register for your Adobe ID, download the e-reader software (Adobe Digital Editions) and then re-open the file downloaded. The file will either be an .ACSM file (DRM protected file which will convert to .EPUB or .PDF when opened) or a non-DRM protected .EPUB or .PDF. If you are unable to find the file, simply access your download link from your confirmation email and re-downlaod it.
If you are on a tablet or IOS device (like iPad or iPhone) simply register for an Adobe ID, download the proper application, and re-download the eBook file from your confirmation email. Once installed your device will prompt you to open your new eBook with the newly installed application.