Hairworks Help!

Greetings . I am in need of some help with Hairworks , i have a big scene in which a rock rolls upon a patch of tall grass . My problem is that i cant get the hair (grass) to collide with the rock at all. This is my first time working with Hairworks so if the question is really mudane i am sorry in advance . Thank you for your time :) ( i work with 3ds max )