The cable can be bought on ebay (dont get the overpriced metra/axxess one) and the firmware would be from the axxess website I beleive.
If you check all posts by the user neil201 he managed to get the kit working correctly on a UK car. You will have to sift through various threads to find out how. His posts below:
For anyone wondering, the thread referenced is as follows:
I'm curious as to whether it'd work for me, though, as I have a Metra 99-7503 rather than the 7507.
I'm going to buy a cable online and hope for the best, nonetheless -- I can get a cable for < ~ $10 and mod it so that it *should* work; so there's no real loss if it doesn't work.
I would modify it as follows:
Will keep you guys posted.
edit: FUDGE! Apparently the cable mentioned in that link is NOT able to update metra kits. I bought mine elsewhere (I wanted it quick, so paid a little extra and grabbed a generic one from Australia (I'm from Melbourne, AU) based on the PL2303HX), so hopefully mine works! If not, it won't be completely useless, as I'll use it for other things I may need to program in the future.
For the record, I purchased this adapter:
(I know it's a bit more expensive than the ones from HK, but I want it asap, so I'm happy to pay that little bit more.)