I'm BA silver, just booked a redemption thru BA website on Cathay from HKG-tokyo in economy.

According to what I've read as a silver I should be able to reserve my seat, but MMB in BA is bringing up nothing, I've tried MMB on the cathay site and its says I'm not eligible. I haven't signed up to the cathay frequent flyer scheme. Am I missing a trick? Is there a way of doing it online anywhere or do I phone cathay?