You can copy the template default/template/paypal/express/review.phtml
to your theme and modify it.
1. To just preselect a shipping method add at the end of the file:
<script type="text/javascript">
//<![CDATA[
jQuery("#shipping_method").val('flatrate_flatrate');
//]]>
</script>
Where 'flatrate_flatrate
' is the shipping method value you want to preselect
2. Tell the customer to select a shipping method [better solution]
In my opinion a better solution is to tell the customer to select a shipping method.
Add a little hint above the "Place Order" button
<div id="missing_shipping_method" style="display:none; padding: 5px; color: #ff0000; float:right;">
Please select a shipping method.
</div>
Add a little javascript snippet at the very end of the review.phtml to control the hint box and add/remove a red border from the select box
<script type="text/javascript">
//<![CDATA[
jQuery('#shipping_method').change(function() {
if (jQuery("#shipping_method").val() === "") {
jQuery("#missing_shipping_method").show();
jQuery("#shipping_method").css({
'border': '2px solid',
'border-color': '#ff0000'
});
} else {
jQuery('#missing_shipping_method').hide();
jQuery("#shipping_method").css({
'border': '0px'
});
}
});
jQuery(document).ready(function() {
jQuery('#shipping_method').trigger('change');
});
//]]>
</script>