When I attempt to import my CR2 (RAW) files from a card reader into Photoshop Elements 15, when I "open"/"open in Camera Raw", I just see file numbers. I would like to see the thumbnails, from which I could select the images upon which I'd like to enhance.
When you say you are importing the files, do you mean into the Organizer or the Editor?
If you are using a newer camera, the raw files may not be supported by Elements 15. Take a look here for the version of Camera Raw that is required by your camera. Elements 15 only supports up to ACR version 9.10.
If your camera's raw files are supported, then I would recommend using the Organizer to view the thumbnails of files that you want to edit. If you want to use your OS to view thumbnails of your files, then you should use Canon's software which I believe will add a file viewer for your raw files, including a thumbnail viewer. If you are a Windows user, there is a free utility called SageThumbs that usually works for raw files.