If you neglected to save the ISO file you downloaded, and thus do not have the DVD, there are 2 methods to get the DVD.
Both involve the receipt that MS emailed to you
First: Near the top is a link to download the ISO file using your key:
If you need to download Windows, write down your new product key and enter it here.
You will need to click the here in your email. The one I included above does not work as a link.
Second: To order a DVD for $15 there is a link near the bottom of the email receipt:
To always have the software as physical backup media at hand, order your personal backup DVD now.