The problem is that when a new camera (with a new RAW file type) is released Adobe only update the current version of Photoshop, PS Elements and Lightroom. When the 6D was released Adobe were on version 6 of PhotoshopCS so they will update that to to read the new files, but won't issue an update for v5, 4, 3 etc.
In addition to the solutions mentioned above (buy an upgrade for PS, buy Lightroom or use Canon DPP) you can also use a special program that Adobe issue free of charge - called Adobe DNG converter for windows or for Mac. This program will convert 6D RAW files to DNG format which CS5 can handle.
Having said that I would also suggest that you at least download the 30 trial of Lightroom 4. The RAW processing engine in LR4 (and CS6) has seen some major improvements over the earlier ACR (Adobe Camera RAW) that is in your version of Photoshop CS.