Even though I've done it many times, I still have trouble remembering how to do this. LOL
Go to your "Account" - which will be the top left hand corner of your screen.
Then notice the bar that goes down the left hand side of the page - it has loads of options.
Scroll down past "Dashboard" until you get to "Settings", and under that you'll see a lot of options. The 10th option down should be "My favorite products". Click on that, and you'll be able to add them.
Hope this helps!