Can you use Photoshop CS?

I have an old version of Photoshop, just "CS." Can I use that in PP?

you could... but depending on the camera model, you will most likely not have RAW ACR support.

I checked. It only goes up to 20d. Thanks.

You can download the free DNG converter from Adobe and convert your CR2 files to DNG. That's still a full raw format, and you can open them in CS Camera Raw.

Or you can do as many of us do by choice: process the raw image in Canon's DPP (which you have already) and use CS for any further editing.

I use DPP instead of CS4 because I prefer the colors I get with it.

-js