How do add additional checkout fields to packing slip?

You will need to edit
wp-content/plugins/wp-e-commerce/wpsc-admin/includes/purchase-logs-page/packing-slip.php

The following code should do it.

$purchlogitem = new wpsc_purchaselogs_items( $this->log_id );
      if ( ! empty($purchlogitem->additional_fields ) ) {
		  foreach( (array) $purchlogitem->additional_fields as $value ) {
			 $value['value'] = maybe_unserialize ($value['value'] );
			 if( is_array( $value['value'] ) ) {
			 ?>
				<p><strong><?php echo $value['name']; ?>:</strong><br /> <?php echo nl2br(implode( stripslashes( $value['value'] ), ',' )); ?></p>
			 <?php
			 }else{
			 ?>
				<p><strong><?php echo $value['name']; ?>:</strong><br /> <?php echo nl2br(stripslashes( $value['value'] )); ?></p>
			 <?php
			 }
		  }
      }

You may need to tweak it a bit. I added the line breaks and nl2br function because it fit my needs.

Also, remember that if you edit that core WP E-Commerce file, it will get wiped out whenever you do an update, so keep a backup.