Answer by karl_

Typically people won't just write scripts for you, but here is the documentation for what you're trying to do. Place this after an OnTriggerEnter or OnColliderEnter for your cube.

http://unity3d.com/support/documentation/ScriptReference/Application.LoadLevel.html

Also, here's the docs for Trigger enters and Collision enter:

http://unity3d.com/support/documentation/ScriptReference/Collider.OnTriggerEnter.html

http://unity3d.com/support/documentation/ScriptReference/Collider.OnCollisionEnter.html

Edit: Alternatively, Fishman92 has answered this question exactly here:

http://answers.unity3d.com/questions/8196/a-game-with-multiple-levels

as well as one I answered:

http://answers.unity3d.com/questions/9097/move-to-next-level-on-collide