You need to use vec_for_screen to convert the coordinates. Sample:
Code
#include <acknex.h>
#include <default.c>


ENTITY* element_onscreen =
{
	type = "CUBE.mdl";
	layer = 10;
	x = 750;
	y = -330;
	z = -10;
	flags2 = SHOW;
}

function main()
{

	video_mode = 10;
	video_window(NULL,NULL,0,"Demo");
	level_load(NULL);
	mouse_pointer=2; 
	mouse_mode = 4;
	
	VECTOR v;
	while(1)
	{
		element_onscreen.pan += 1;
		if(mouse_left == 1)
		{	
			v.x = mouse_pos.x;
			v.y = mouse_pos.y;
			v.z = 750;//(element_onscreen.x)
			
			//convert coordinates
			vec_for_screen(v,camera);
			vec_set(element_onscreen.x,v);
		} 
		wait(1);
	}
}