Help! I installed my NIS2011 using the disc but my other PC's disc drive won't open

In my account it says accurately that I have one more key (I am so bad with computer speak, please bear with me) but when I tried to download it from the site it wanted me to purchase another key. How can I download the last one when I used the disc for my 2 other PC's without having to pay again. Please talk in layman's terms.....and thank you so much! Also I am new to Norton.