The marketplace and ecommerce features are now available to accounts "basic" and above.
Please note, new images added to the marketplace may not appear for a few days. These items will still be available through your MuseCube site in the meantime.
Be sure you are assocating categories and keywords to any new images that you are adding to the marketplace.
When the marketplace is converted to use the new categories (within the next week), these new items will appear.