You may probably have to download the update for the Camera Codec Pack for Win 8.1 from here: http://www.microsoft.com/en-us/download/details.aspx?id=40309 as support or the CR2 files should be there.
It shouldn't make a difference if the Codec isn't installed as the CR2 files should still show in the browser as it will in the OS but no thumbnails.
In Preferences General, if you select "Treat JPEG Files next to RAW files as separate photos" you will then see all the CR2/JPG files, if it isn't selected you will only see the CR2 (but without the CODEC no thumbnails) but it will import both the CR2 and JPEG files.
I use both 5D Mk III and 7D and have no issues except the fact that the thumbnails were not displayed until I installed the CODEC pack.