While the Mission Control preference pane allows you to map common keyboard shortcuts to Mission Control actions, its interface doesn't allow you to map more esoteric keys like F19. Here's a quick way to assign an unlisted keyboard shortcut to one of the Mission Control actions.
Go to the Keyboard preference pane, then the Keyboard Shortcuts tab, select Mission Control and enter your keyboard shortcut there. It will automatically be reflected in the Mission Control preference pane. No need to muck around with Terminal commands!
[kirkmc adds: This is a good reminder that other features that offer a limited set of keyboard shortcuts may be available to change in the Keyboard Shortcuts pane of the Keyboard preferences. For example, the Spotlight preference pane offers a number of shortcuts, but you may want something other than what's in its menu. You can set any shortcut you want in the Keyboard preference pane.]
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