Magento – How to get the selected custom options of a item in wishlist

Look at the loop code of sidebar, by default it's:

<?php foreach ($this->getWishlistItems() as $_item): ?>

Inside thip loop $_item->getManufacturer() will return your attribute value, but you have to be sure, that product have this attribute fulfilled.

You can display it i.e. like this:

<?php if ( $_item->getManufacturer() ): ?>
Manufacturer: <?php echo $this->htmlEscape($_item->getManufacturer()) ?>
<?php endif ?>

Edit:

Which Magento version you're using? Probably this code will work, but its not optimal:

<?php if ( $manufacturer = Mage::getModel('catalog/product')->load($_item->getId())->getAttributeText('manufacturer') ): ?>
    Manufacturer: <?php echo $this->htmlEscape( $manufacturer ) ?>
<?php endif ?>

You can try to replicate the way Magento shows the options in the order view page.
Here goes:

$items = $order->getAllItems();
//or you can use $order->getAllVisibleItems();
//create an instance of the item renderer block.
$block = Mage::app()->getLayout()->createBlock('sales/order_item_renderer_default');
foreach ($items as $item) {
    //attach the item to the block
    $block->setItem($item);
    //get the options 
    $_options = $block->getItemOptions();
    foreach ($_options as $option) {
        $label = $option['label'];
        $textValue = $option['print_value'];
        //do something with $label and $textValue
    }
}