See attached screenshot for a visual explanation.

The <header> area in which the "Add User" button is contained is floated. As such, the <header> has no height, and the <form> element overlaps it. Consequentially, the button's clickable area is obstructed by the overlap.

Ensuring the contents of the <header> area affect its height should resolve the problem.

Adding either a <br class="clear"/> or adding an overflow:auto; CSS style to the <header> will allow floated contents to affect the height of the <header>, thus solve the problem.