<div class="mm_vert_container" style="display: block;"></div>

<div class="mm_vert_container" style="display: block;">
    <div>A</div>
    <div>B</div>
    <div>C</div>
    <div>D</div>
    <div>E</div>
    <div>F</div>
</div>

<div class="mm_vert_container" style="display: block;">
    <div>A</div>
    <div>B</div>
    <div class="active">C</div>
    <div>D</div>
    <div>E</div>
    <div>F</div>
</div>

$(".mm_vert_container div:nth-child(2)").addClass("active");