The rending window in the Creation kit don't show any trees, very few buildings (only major cities are shown, both centered and in distance), even fewer rocks or water. All I can see is the terain itself. I was trying to add a simple bridge to an area just outside Riften (39,-22). Inside the game the area got some trees, rocks, a patch of seashore and a nearby farm. But nothing of that can be seen in the rendering window, and while ading the bridge it self seemed to work in the render window, the only thing that showed up in the plug-in was a new path I painted on the terain.

If there is anyone who know how to solve this I would be so greatful for your help.

This is a common issue. Try pressing F5.

I have done that, several times, doesen't work.

UPDATE: Problem solved. I was trying to add the bridge in "RiftenWorld", instead I needed to be in "Tamirel"