Okay, I also tried the suggestions above. Firstly I tried resetting the computer and overture would still not load. (no trouble loading black plague or requiem though).
Secondly, as also suggested by urkle, I followed the instructions at
http://www.thexlab.com/faqs/resetlaunchservices.html
The game loaded!! However, there is a catch. I exited the game and tried to load the game again and got the same message again that "you cannot open application because it may be damaged or incomplete". I tried resetting computer again, but this did nothing. Following the steps in the link only works for the first time I launch the game after reset. It appears that I need to follow the steps again from the link above and reset the computer to get the game game to load again. Hopefully there is a more permanent solution to the problem. I don't fancy having to delete files and reset my computer every time I want to launch this game.
I am running the game on a MacBook Pro 2.4ghz Core 2 Duo, 2gb DDR3 ram, Geforce 9600M GT using OS 10.5.7.
Oh yeah, I also tried removing the game folder and reinstalling as also suggested, to no avail.