Not sure if this will help, but I just added a clickable link to my pixels site in one of the sidebar widgets. I'm not sure I even knew there was a specific FAA shopping cart widget for WP. The way I did it was the easiest way for a technologically challenged person such as myself.
I tried to install the widget some time back and couldn't do it. When I contacted WordPress they told me I had to have the next tier of service to use widgets. It's been a while, so I'll try Abbie's instruction above and see if I can get it to work now.
Thank you all for the help. It sounds so easy and yet as I try - it's not working. When I add the code one of two things happens when I click preview - either only the text of the code shows up or nothing shows up at all. I'm not sure what I'm doing wrong, but I'll keep trying.