@Chrispiz, all entities, objects etc have properties which are accessed via right clicking.
Select the terrain, right click and on the first panel you will see two check boxes, albedo and ambient.
-100 for ambient will get it as dark as it can go and as Plouf suggested, the albedo may give results too.
So if your not sure with action scripts just yet, apply the check box settings in the properties of each object.
Cheers.