Your code is correct you just used .div
instead of div
HTML
<div class="ui grid container">
<div class="ui center aligned three column grid">
<div class="column">
</div>
<div class="column">
</div>
</div>
CSS
div{
position: absolute;
top: 50%;
left: 50%;
margin-top: -50px;
margin-left: -50px;
width: 100px;
height: 100px;
}
Check out this Fiddle