The patch checks the registry to find the install path. Have you restored the registry since installing the game, or ran any registry cleaning programs that might have been a little aggressive?
Check for the registry path
HKEY_LOCAL_MACHINE\Software\CDV Software Entertainment AG\Divine Divinity
If it doesn't exist, save the reg file from the manual install post Kiya linked to, edit it so it lists your install path, and merge it into the registry (right click and merge).
Alternately, you could try just re-installing the game, though backup your saved games folder first, just in case.
