That's what I've been doing, but it only shows me the folders in the internal memory. I can't see how I'm supposed to select the files on the SD card; that is, the card doesn't show up in rbutil as a folder you can select.
You mean the SD card doesn't show up in Windows (or whatever operating system you're using)? If so, you'll need to figure out why that is before you can install things to it

Edit: Rereading this I suspect you've picked the wrong mount point for your device. Again, I've never actually tried this so I could be completely wrong, but I believe if you want to install something to your SD card, the mount point becomes the SD card. My guess is that you're selecting some other device, hence you can't select folders on the SD card.