Photoshop menu File>Open select the CR2 file. If you have a resent canon you may need CS6 or newer only they support ACR 8.6 beta with RAW support for the latest cameras. If you have an older version of Photoshop you may need to create Adobe DNG file for you CS2 file using Adobe DNG converter and process the DNG files with you Photoshop versions older ACR support.
Which version of Photoshop?
Which Canon camera model is the CR2 file from?