$(document).keypress(function(e){
    switch (e.keyCode) {
        case 40:    // down
            n = $('.screen-wrapper').next()
            $.scrollTo( n, 800 );
          break;
        case 38:    // up

          break;
        case 37:    // left

          break;
        case 39:    // right

          break;

    }
});

<div class='screen-wrapper'>
<div class='screen'>
    <div class="sections">
        <ul>
            <li><img src="images/portfolio/sushii-1.png " /></li>
            <li><img src="images/portfolio/sushii-2.png" /></li>
            <li><img src="images/portfolio/sushii-3.png" /></li>
        </ul>
    </div>

    <div class="next"></div>
    <div class="prev"></div>
</div>

n = $('.screen-wrapper').next()

var s = $('.screen');
          var curr=-1, node;

          $(document).keypress(function(e){
             switch( e.keyCode ) {
               case 40:
                  node = s[++curr];
                  if (node) {
                    $.scrollTo( node,800);
                  }
                  else {
                    curr = s.length-1;
                  }
                  break;
                case 38:
                   node = s[--curr];
                   if (node) {
                     $.scrollTo( node ,800);
                   }
                   else {
                      curr = 0;
                    }
                   break;
                }
          });

window.location.href = '#section-' + curr;