you need to go to the “unknown sources” section, as seen in this image:
in the new menu it is more hidden, as explained in this video.
great game! very tough and unforgiving, but fast-paced, humorous and fun! the first game that made me crouch and almost crawl on my living room’s floor. 😄
i love the “name your own price” policy, as it allows to first test the game before paying.
i could not make voice chat work in multiplayer mode. is it implemented? if not, it would be very useful to have.