HAve you actually paid for the item?
Your seller should contact you to arrange collection and you should be able to reply to that message.
If they don't and ou cannot contact the seller, if they are a business seller their contact details will be shown at the bottom of the listing, if they are private, then you may need to contact CS to ascertain or allow you to contact the seller to arrange collection.
If you pull the listing up on to your screen there should always be a "contact Seller" tab to click on.