I ended up getting a little farther by editing the file:
/etc/tvheadend/dvbmuxes/_dev_dvb_adapter0_Samsung_S5H1411_QAM_8VSB_Frontend
and changing the constellation value to "8VSB", since QAM-64 and QAM-256 were the only things that showed up for me to select. I restarted tvheadend after making that change. Now, I can now click "Map DVB services to channels" and it scans now. I'm assuming I now have to add channels. I'll read up on that next.