Comment 22 for bug 1302462

What you need to do is edit /etc/xdg/autostart/nm-applet.desktop with your favorite text editor as admin.
Go down to the Exec line and change the entry from nm-applet to dbus-launch nm-applet.
Go to the NoDisplay=true line and change to NoDisplay=false.
Save the file and reboot!

from http://askubuntu.com/questions/449658/networkmanager-tray-nm-applet-is-gone-after-upgrade-to-14-04-trusty