head over to http://www.spacemarine.com/
and register yourself an account.
After activating it, log in and head over to http://www.spacemarine.com/demo
which gives you an early access code, pop that to the "Activate a Product on Steam"
thingy in Steam library and it starts to download it.
Warhammer 40k Space Marine Demo Playthrough Part 1 [HD]