Most Canon DSLRs use CR2 extension for their raw files, so you need to specify which camera it is coming from.
Then take a look at the Camera RAW version needed: Camera Raw plug-in | Supported cameras and the compatible Photoshop version: Camera Raw-compatible Adobe applications
If the camera RAW version needed is higher than what is supported by CS5, you either need to upgrade Photoshop or use the newest DNG converter to produce a DNG file consumable by your version.