You have to be following an account to add it to a list. Go to the account page in question and make sure you're following them. Then click the 3-dot menu to the right of their name and click the "Add or Remove from lists" option. From the popup that appears next, you can add that account to existing lists or create a new list.
To view/manage your lists, go to your own profile page then click the 3-dot menu and select the "Lists" option.
Hope this helps!