Snayler is correct here, a friend of mine was playing around with ettercap at university and accidently took on a whole university subnet of approx 150 hosts and caused a DOS attack against a switchYou shouldn't select all clients, that's a mistake.
-editing the etter.conf file is essential as it tells ettercap what html tags to look for when sniffing the network
-also setting up iptables appropriately sometimes will solve some issues
/usr/local/etc/etter.conf [uncomment the iptables]
echo "1" > /proc/sys/net/ipv4/ip_forward [enter this into shell]
A great tutorial on ettercap can be found at http://infinityexists.com/