I've got Metastock Pro FX & and Quote Center with the forex data feed.
I'd like to suscribe to futures data and I think that "QuoteCenter Worldwide Futures, Commodities & Derivatives Data" would suit my needs.

But when I click on the "order now" button, I'm redirected to the "order QuoteCenter Add-Ons" page.  I click on "Continue" and it tells me that my cart is empty...

Is that because I've got Metastock FX and not Metastock Pro ?

It's because the Forex package is a part of every other package.  You would want to cancel your ($99) Forex package and simply switch to the ($135) QC Worldwide Futures package, which will include Forex.

Thank you Justin !

And how do I do that ?  I just used the Technical Support -> Submit Email Request, but is that enough ?

It is best to go directly to sales@equis.com for subscription changes, but the method you used should also work.