How do you get left 4 dead sacrifice

You need to specify which gaming system you're talking about.
On the Xbox 360, you'll need to go to the dashboard. From there go to "Game Marketplace" then find the option to search by letters "A-Z" then select the letter "L" and find "Left 4 Dead 2". Select it and find "Downloads", from there look through the list until you find the "Sacrifice" DLC. Then simply purchase the download and once complete, start up your game.

As for other gaming consoles, I'm not sure. On the PS3 it would be something similar to the above.