Authorisation.

I wish to authorise my computer in order to open .acsm ebooks to read on my Nook. The instructions tell me to 'Click on library, and from the drop-down, select 'Authorise'. When I click library, I do not get this drop down. I can only find 'Authorise' on the Help button. When I click Authorise, I receive an error message. What should I do now?