Iphone responsiveness not working

Dear Sir,

When I am opening my website http://www.mensahero.com.ph in my desktop or laptop the top navigation menu are working fine (they are visible – Home, About, Editorial, etc.), however, when I am opening my site in my iphone and other smarthphone devices, these top navigation menu are not working and they are not visible but we see only 3 lines (compressed like a bar). Please help to resolve this problem.