Open CS2 and go Help > About plugin > Camera RAW and see what version you have. CS2 supports ACR 3, which should be OK for the original 5D.
So make sure you have all available updates. I am not sure how you go about that with CS2 because you now have to use the special version because the original servers are no longer working.
Thank you for your answer, I have Version 3.0 so sounds like I need to update?
Do you know which update I should download? The latest RAW?
is the latest for CS2. I know you are running Windows, because Intel Macs do not support CS2.
That only covers the first model. If you have the Mark II onward, let us know, because you will need the dng converter.