Add "search box" to menu bar [mystique theme]

Hi everybody, i really need help.
I searched for 5 hours for what i want but i don’t found any solution.
Here is my web page : http://pleinsdetutosenvrac.fr
I have a menu bar (Accueil, Nous contacter, A propos) and in this menu bar (on the right) , i want to add a “search box”. [so the visitors can search articles].
I tried many solutions but nothing work with me.
I have the Mystique theme 3.3.2 by digitalnature.
Then please try to help me to add this “search box” to my menu bar (on the right).
[i tried many solutions, like adding code to functions.php etc…, but nothing works].
Thank you very much.
Alexander