This has happened before, last year I noticed several power packs just seemed to have disappeared, a few days later and they were back. You could just order it from hobbytown or other store that carries FT products.
FYI: Here's the offical response from FT/Amain customer service:
"The power pack c for dual engine planes has been discontinued. However, we are currently waiting to get the new replacements for that pack.The replacements should be added to the site and available for purchase sometime within the next week. If you have any further questions, please let me know. "