Penumbra overture instant fail!

[acabedo]

Just re-ownloaded Penumbra from Gamersgate, (originally bought a few years ago), seems to have downloaded fine, when I try to start the game the screen goes black for a second then it quits (I think, my screen just reverts to whatever I had open before). I have an imac,

El Capitan Ver: 10.11.3 (Late 2012)
Processor 2.9GHz Intel Core i5
Memory 8 GB 1600 MHz DDR3
Graphics NVIDIA GeForce GT 650M 512 MB
serial CO2JT3WRDNCT

I've looked everywhere but I can't find a solution, any ideas?