Are you sure you have included both Cycle2's js and css files? And that you are using existing images and correct paths? There's nothing wrong with the code you posted, you can check it here: http://jsfiddle.net/natnaydenova/x4dfv/
Somewhere in your HEAD section:
<link rel="stylesheet" href="css/cycle2.css">
<script src="js/jquery-2.1.0.min.js"></script>
<script src="js/jquery.cycle2.min"></script>
And then in your BODY:
<div id="slideshow" class="cycle-slideshow" data-cycle-fx="scrollHorz" data-cycle-pause-on-hover="true" data-cycle-speed="200" data-cycle-log="false" >
<img src="http://malsup.github.io/images/p1.jpg" class="first" />
<img src="http://malsup.github.io/images/p2.jpg" />
<img src="http://malsup.github.io/images/p3.jpg" />
<img src="http://malsup.github.io/images/p4.jpg" />
</div>
<div class="center">
<button data-cycle-cmd="prev"><</button>
<button data-cycle-cmd="next">></button>
</div>