SometimesDigital Editions gets device registration a little confused.
Try explicitly unregistering the Kobo (ctrl-shift-E), unplugging it, and plugging it back in again.
It may then get recognized as an unregistered device, and give you the chance to register again.
If Digital Editions is very confused, you may need to go to file explorer on the PC/Mac with the Kobo plugged in,
go to the drive that is the Kobo, and delete the directory 'Digital Editions' from the Kobo.
Then restart Digital Editions and Kobo registration may be able to work.
I have the same problem, but my computer does not recognize my Kobo Glo either, so I am not able to delete the directory "Digital Editions" from the Kobo folder. I purchased a new Kobo book yesterday and it appears in my Kobo Desktop but not on my Kobo itself???
Someone recently posted that the Nook will not work on a USB hub, but will work if plugged directly into a computer USB port.
I don't know if this applies to the Kobo Glo, but worth a try.
This was indeed the problem with my Kobo Glo ... working fine now ... thanks.