Log in /out, register button

Hy all,
I have Asteroid theme installed for my website (a child theme obviously) with S2member plugin. It is default login link in side menu, but how can I add login/register area to the main menu (Reason: I use full width pages with only main horisontal menu bar at the top of the page).

here is my intention:

https://muntesco.byethost4.com/wp-content/uploads/2015/09/login..jpg