How do I position a div at the bottom center of the screen

align="center" has no effect.

Since you have position:absolute, I would recommend positioning it 50% from the left and then subtracting half of its width from its left margin.

#manipulate {
    position:absolute;
    width:300px;
    height:300px;
    background:#063;
    bottom:0px;
    right:25%;
    left:50%;
    margin-left:-150px;
}

If you aren't comfortable with using negative margins, check this out.

HTML -

<div>
 Your Text
</div>

CSS -

div {
  position: fixed;
  left: 50%;
  bottom: 20px;
  transform: translate(-50%, -50%);
  margin: 0 auto;
}

Especially useful when you don't know the width of the div.