It this a dedicated kobo eReader device or kobo tablet? If tablet, which eReader app are you trying to use?
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If a dedicated eReader device, you must register it on ADE (Adobe Digital Editions) while it is plugged into the computer by USB.
Usually, ADE will automatically prompt you to register the device if it is plugged in when ADE starts.
If not, there should be be an item like 'Authorize Device' in the Library menu dropdown.
ADE 2.0 is stroppy with some devices.
If ADE recognized the device when plugged in with USB it will show up in the left panel in the ADE Library view.
If it doesn't, try ADE1.7.2 which is more reliable than ADE2.0.
Version 1.7.2, it is a little difficult to find, available on Adobe site for Windows and for Mac.
http://helpx.adobe.com/digital-editions/kb/cant-install-digital-editio ns.html
The forum software is sometimes corrupting the link above. There shouldn't be a blank in 'editio ns.html'. The following redirects to the same page: http://tinyurl.com/diged172
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If a kobo tablet, please post back with more details about the tablet and the eReader app you are using.