Support for the 5D Mark III was added in Lightroom 4.1. What version of Lightroom for are you running? Unless you are running the original installation from the disk and haven't updated, you should be able to open the raw files from that camera. Please clarify what you mean by wanting to import from the drive. All of your image files are simply in folders on your hard drive.
I will have to check the version when I get back to my other computer. I'm assuming that I do not have the more recent update installed. I'm planning to just get LR CC anyways.
1. How can I now go back and import all these .CR2 files from the drive without knowing which folders they are in? I have lots of subfolders inside of the folder that I just imported.
In the import dialog, right-click on that topmost folder and choose to include subfolders.