When my level is load,i place my trees and other stuff on terrain. How i can make for user don`t see this.

Create a dummy panel as a loading screen that is the full size of the screen.
That way the user cant see the loading happening behind the panel.
That what you mean?

Yeaa!

and then call this in the loading function (the one placing the trees) when done loading:
loading = 0;
you may have to exchange the vector(screen_size.x,screen_size.y,0) and the NULL parameter after it, if it doesn't work. I don't remember if it was the scale or the size that needs to be set.
check out the draw_quad function in the manual.