ArgumentError: Error #2025 - I can not removeChild?

Your problem likely lies in declaring the object inside a function.  It will only have scope within that function when you do that.  I see where you create a new instance of the object in the function where I assume you are trying to remove the first one... the second one is the only one seen at that point, and it has not been added.  Try the following:

var urunlermenum:urunlermenu; // declare it outside any function

function urunlergelsin (e:MouseEvent):void {
    urunlermenum = new urunlermenu();
    bg.solurunmenusu.addChild(urunlermenum); // Added new one
    var geridonbuton:geridon = new geridon();
    bg.geridonus_mc.addChild(geridonbuton);
    bg.geridonus_mc.addEventListener(MouseEvent.MOUSE_DOWN, geridon_f);
    }

function geridon_f(e:MouseEvent):void {
    bg.urunbtn.removeEventListener(MouseEvent.MOUSE_DOWN, urunlergelsin);
    bg.geridonus_mc.removeEventListener(MouseEvent.MOUSE_DOWN, geridon_f);

    var urunlermenum:urunlermenu = new urunlermenu();  // remove this
    bg.solurunmenusu.removeChild(urunlermenum);

}