No permission to copy book

I am unable to transfer my first eBook purchase to my Kobo Glo eReader.  When I connect the Kobo device it does appear on the left side when I open Digital Editions and the book I purchased is there but when I try to drag it to the eReader, I get error message "No permission to copy the book".  I'm guessing this is becaise I did not "validate/authorise" at time of purchase as there was nothing applicable in the drop down "vendor" list and now I can only read my book on my computer.  Can I change this validation/authorisation now or do I have to re-purchase the eBook and what should I use from the "vendor" drop down list? Can someone please help.  I am only new to all this.