I installed Kies on my other iMac, from a different Samsung website.
I used that same dmg file on my other iMac using file sharing, and voila, success.
So, the update that came through automatically from Kies was at fault (I am in Indonesia at the moment), as was the download from the UK Samsung site.
The problem had to be with the actual application I was downloading. Once I switched to the Australian Samsung website, all was working again.
I hope this helps someone out there. I have been doing my head in for 3 hours now!