Bioshock 1 won't start.

Hi forum members,

feral interactive is rather slow on support so I thought someone helpful might know the answer to this issue. My system:

MBP, 2.2 GHZ Core Duo

2GB Ram

NVIDIA GeForce 9400M

OSX 10.5.8.

I know it is not the newest machine, but it meets (os) or exceeds (hardware) all the requirements to play. in fact, I played the demo and it ran smooth as butter. That actually made me buy the game. So I paid, dl'd and installed the game.

the problem: I cannot start the game!

the game icon is overlayed by a grey shadow, and when I double click it this message appears: "this version of osx cannot start this version of the programm "bioshock" " (this is an approximation, because it actually appears in german). So what is up with that? I am 100% certain that I am on 10.5.8. (I just looked it up again), the demo runs nicely, but the game doesn't. what to do? thanks for any input! (any I would prefer to stay on 10.5.8 for now).