How to make an animation in the animation editor loop?

I'm trying to make an idle animation or any animation can can loop and i finished the idle animation but i don't know how to make it loop and i try to search on youtube how to do it and there's a loop button : http://prntscr.com/pf9gmt . But in my animation editor it looks like this : http://prntscr.com/pf9gya . And i really don't understand why it's not there. Unless it's inside a different option then i don't know which one it might be. Please tell me.