The update is an automatic download when the Apple Software Update runs. The Digital Camera Raw Compatibility Update 2.4 is a small 4MB download and installs without the need for a reboot. If you need to download it, click on this link.
If you have already imported your CR2 raw files into Aperture, prior to the update, you will need perform the migration function in oder to properly view these files.
In Aperture, do a search for or just select all your .CR2 files. Once selected, go to the FILE pulldown menu and select the MIGRATE IMAGES option. If you are not a power use, just use the default option on the first dialog box that comes up. Another dialog box will/may appear if you have selected files other than your .CR2 raw files. Make sure you have selected to upgrade your RAW files. Non-RAW files will not be affected by the migration.