I'm working on a mac.
I could open my (canon eos 5d mark II) CR2 raw files without issues before christmas, now however I get the "Could not complete your request because Photoshop does not recognize this filetype" error.
I tried updating my raw plugin, both through package download and through photoshop, but the problem still remains.
My plugin version seems to be 188.8.131.52 if that helps at all.
You need at least 5.2 for the 5D mk2. Not sure how you managed to drop a couple of versions, as most people can't even do that if they try!
The best version of ACR for CS4 is 5.7. You can download it here http://www.adobe.com/support/downloads/detail.jsp?ftpID=4680
I thought it was weird, since I've downloaded the version you linked to, and I've tried installing it several times earlier today. It seems to be working now though!