Recovering iWork from backup
You also need to recover library files:

/Library/Application Support/iWork'09
/Library/Preferences/com.apple.iWork09.installer.plist
/Library/Preferences/com.apple.iWork09.plist


In my case, this was the minimum requirement to get Keynote starting again. 

Additionally, you might want to recover user-specific files from your user directory:

~/Library/Application Support/iWork
~/Library/Preferences/com.apple.iWork.*