Just a few things to check..
Do you have ADE installed and Sony Library installed on the computer?
Do you have the exact same password for ADE and Sony Library?
On your Reader, on the main menu: under Settings, under About: does it show that it is authorized to the Sony store "Authorized for the ebook Store" and next to the Adobe logo: "This device is authorized for Adobe ADE protected content" ?
Log in to the Sony Store: go to "My account". Scroll down to the bottom. Under " My Authorized Computers and Devices". Is that computer and device listed?