I just bought a product key today at Staples and every time I try to register it, it keeps saying it is an invalid key. What am I suppose to do? 

Hi,

this article should help:
http://office.microsoft.com/en-au/access-help/find-your-product-key-for-office-2013-and-office-365-suites-HA102830243.aspx?CTT=1

or:
https://officesetup.getmicrosoftkey.com/OfficeSetup/Home/SupportF