Support for the Canon EOS 6D first arrived in Camera Raw 7.3 - only Photoshop CS6+ is compatible with Raw 7.3
You have two choices
- Upgrade to CS6, or
- Download the free DNG converter from the Adobe website, convert all your Raw files to DNG and edit the DNGs in Photoshop CS5.1
I purchased the Adobe Creative Suite 5.5, which had Photoshop CS5.1 on the 28th March 2012, am I eligible for a free upgrade to CS6? And if not how much will an upgrade cost?
You'd have to ask Adobe to be sure, but I doubt that they'll offer you anything for free now that you've waited this long after the release of Photoshop CS6. But definitely ask.
This page gives info on upgrades: