Where is force quit on Ubuntu 11.10?

The command, as WarriorIng64 already said, is xkill.

To show some instructions for the end user, maybe use

notify-send "Click on an application to force-close it, or right-click to cancel."

Now to combine two commands into one launcher, you'll have to wrap them in sh -c '...', so your .desktop file should say

Exec=sh -c 'notify-send "Click on an application to force-close it, or right-click to cancel."; xkill'

Instead of writing your own you can try this:

Go here and check it out; http://www.omgubuntu.co.uk/2011/10/freezeunfreeze-unity-app-killer/

or http://www.omgubuntu.co.uk/2011/10/force-quit-applet-unity-launcher/

I don't know if there is an icon for force quit, but the relevant command is xkill.