To choose your recipes, you must first select the Custom box, which allows you to choose from all our selection every single week.
If it's your first order, you'll be able to select your meals at the bottom of the Delivery Preferences page, after choosing the Custom box, proceeding to "Checkout" and entering your e-mail address.
When you're on your account, you can simply click on "My Deliveries" on the top left of your screen, then click on the week of your next delivery on the calendar to your left. After that, you can click on "manage this box" on the right hand side of your screen, besides the "skip" button. Please note that you'll have to unselect meals before you'll be able to select new ones.