Using the the F3-key should normally activate Exposè/Mission Control. In some circumstances nothing happens when you hit the F3-key or the four finger up gesture.
To solve this problem apply the following defaults write command:
defaults write com.apple.dock mcx-expose-disabled -bool FALSE
Restart the OSX Dock:
killall Dock
F3 and the four finger up gesture will work again.
I cannot thank you enough, this fixed my expose issue.