So what I did is go to my Marketplace Listings in Second Life, where you physically drop the inventory in. You'll find an unassociated folder that's empty without a subfolder where you store the product. I had to relog for this to show up by the way.
It's a bare folder, so you need to create a subfolder (drag/drop a notecard or something with the product name into the folder, then just delete that item). Then you put your stuff in the subfolder with the same name, like it looks like your other listings.
Then set that subfolder to activate.
Then on the Marketplace website set it to Listed, and you're set.
Hope that makes sense.