Looks like on Raspbian Jessie the way you configure a static IP has changed. You used to configure static IPs in /etc/network/interfaces, but on Jessie it looks like the file to edit is now /etc/dhcpcd.conf (per the comment at the top of the prior file).
Scroll to the end of the file and paste something like this, editing IPs to match your network:
interface eth0 static ip_address=192.168.1.xxx/24 static routers=192.168.1.xxx static domain_name_servers=8.8.8.8, 8.8.4.4
Reboot, done.