Click on the Terrain and make sure that, in the Inspector, Terrain Collider->Create Tree Colliders box is unchecked.
answered Apr 19 '12 at 10:10 AM
To avoid going through trees, you need two things:
Have a look at the lerpz tutorial: http://unity3d.com/support/resources/tutorials/3d-platform-game
answered Apr 19 '12 at 07:13 AM