Enable with these instructions:
1) in ENGINE_SETTINGS.XML (found in the DATA folder where you installed Alien:Isolation) change the stereo options to:
<Setting name=”Stereo Mode”>
<Quality name=”Rift” precedence=”4″/>
<Quality name=”Off” precedence=”4″/>
2) Set to extended mode.
3) In the configuration utility pause the service.
4) Start the game.
5) To play the game normally, reactivate the service and set it to direct to rift mode.