Ah, I apologize for my misunderstanding.
Once you have the list created, you'll want to go into your collection and find the card(s) you want to add. As you scroll down on the cards page, you will see an option underneath the "Add to collection" section that shows your list(s), with a clickable "Add to list" link to the right.
Sorry, if I was good with screengrabs I'd have a picture. Hopefully this makes sense and helps.
I'll never quit collecting entirely, but I am downsizing. Check out my COMC store and help me thin out what I don't want so I can buy cards that I do want. See something you like? Send me a message on here, and we can knock the price down quite a bit. I'll even take a bit of a loss if it means getting you a card you really want.