Hi sslangs! Welcome aboard.
Sometimes you just need to pull your SD card out slightly, and then push it back in, and check and see if your S3 then recognizes your card. That worked for me, when I first installed my SD card.
With Kies, I've read and experienced that it can be rather finicky. And honestly, I don't recall if it lets you download to your ext. sd card. Someone will be along to help you with that.
But with my S3, I've been having good success with (if you scroll down, you can click on 'Download iSyncr for PC Desktop App Now' – or 'for Mac') and it's been working great for me. When I plug-in my S3 to my PC, it's instantly recognized as a [SGH-I747] drive, with: [E: Phone] & [F: Card] sub-drives—where you can direct any music, photo's, video's and/or files that you want—onto either your phones' internal SD, or your external SD card.
Plus, the google play store has both: & .
I got tired of messing around w/ Kies and uninstalled it, but like I said, someone more knowledgeable will be along to help you with Kies.