I ran into the same problem with my install, You need a patch if you are using it on a wireless card or you need to manually switch the card into monitor mode so it acts like an ethernet interface. The Pcap loop error happens when you use an interrupt ( Ctrl + C, etc ) and that is because you are forcing the process to quit so that is normal. Make sure you are reciving packets by typing snort -dv and see if you see packets...you should instantly.