so eventually I realised, all on my own & with NO help from this website, that since I'd moved to a new mac, I'd need to re-register the software. the app itself doesn't tell you why "the activation code isn't valid", it just closes when you hit 'ok'. this is not helpful.
so... what's the computer ID? mac address? its WINS name? whatever I've called the hard drive? who knows?
I dare say I'll crack this with a bit more googling, but I've got work to do, & this is wasting my time.
it should NOT be this difficult.