Lauren,
The option to provide the serial number and product code is there for those who do not have their 20 digit license key. To download ActivInspire select your operating system (Windows, Mac, Linux) and in the next window where it asks for the serial number and product code, select the option at the top of the page "If your download does not start automatically click here". Save the download and then Install it. When you open ActivInspire the first time the licensing window will appear where you will input your Name, Organization and the 20digit Activation key.
Hope this helps!