Thank you for the answer!
But if I try it this way, I get an error in "...include\windows.h" in line 6519: syntax error <RECT rcPaint;>!
EDIT: Fixed this by changing order of lines...
But now I get an error after starting the level: EMPTY PROTOTYPE CALLED IN NEWTON_START
What I do in main() is:
//set some level preferences
//load the level
newton_start();
wait(3);
ENTITY* ball = ent_create("kugel.mdl", nullvector, NULL);
newton_addentity(ball, 75, NEWTON_SPHERE, onforceandtorque);
while(1){
newton_update();
wait(1);
}