I'd like to change my Alternate Desktop shortcut in Xubuntu. How do I do that? I don't see the option under Keyboard's Application Shortcuts?
Xubuntu 13.04 has two different sets of keyboard shortcuts,
You're simply in the wrong screen. Close out of Application Shortcuts, that's for starting an application with a keyboard shortcut. What you want is to go under Settings → Window Manager → Keyboard. Then you should be able to set new keyboard shortcuts.