When I import a model as a barbedwire (http://www.snakesolid.org/barbedwire.mdl) into my level map, compile it and run it, the model has strange black spots (all the empty area is filled with black). How can I make this barbedwire get realistic into my map ?

Try checking overlay in your object properties in the level map (WED)

Still get the black spots... It's also with tree's and bushes :S

This has been answered many times before. In order to remove the black spots youll need to use a tga file with alpha transparency. Which will likley lead to sorting errors. In which case youll have to use the Improved overlay shader found Here

I'll give it a try. thanks!