How do I authorize my computer with an Id after having chosen to authorize w/o one?

Hi - I tried to authorize my computer with an id, but it kept rejecting my username and/or password.  Finally I authorized without an id so I could get to the book I want to read.  Now, I'd like to fix this so that my son can read a book on another computer.  Does anyone know how I can do this at this point?  Thanks!

Sarah