Hello Rob,
Outside the full versions of Cubase 7, Nuendo 6 and Wavelab 8, the Mp3 encoder comes only as a demo license with Cubase LE/AI/Elements 6 and Wavelab LE/Elements 7 and Sequel. You will need to purchase the Mp3 upgrade patch to have a full license allowing you to export Mp3’s from inside your application.
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.