You will now have reached the "Select mail pack" screen. Note that you will now have an order reference, which is displayed in order status bar.
You now need to tell us which mail pack to use for your mailing. Mail packs you have already created will show here, or you can go and create a new mail pack to use just for this mailing.
Once you have chosen an existing mail pack (by clicking select for the mail pack you want), or gone away and created a new one, you will then be able to click next to continue.