The Canon Powershot G15 was first supported in Lightroom 4.3
http://helpx.adobe.com/creative-suite/kb/camera-raw-plug-supported-cam eras.html
So you'll either have to upgrade to LR4.4
or convert the Raw files to DNGs using the free DNG converter then edit the DNGs
http://www.adobe.com/products/photoshop/extend.displayTab2.html