Hello Richard,
You need to purchase the Steinberg Mp3 Upgrade Patch:
http://www.steinberg.net/en/shop/buy_product/sku/500242444.html
Purchasing this upgrade will give you a 32 character activation code to download the license to your eLicenser will allow you the ability to export in the Audio Mixdown window using the MPEG 1 Layer-3 format which is the same as the Mp3 format.