Magento – how to display recently viewed products in cms block and call it in details page

Update your file as per below code

<?php if ($_products = $this->getRecentlyViewedProducts()): ?>
<div class="block block-list block-viewed">
    <div class="block-title">
        <strong><span><?php echo $this->__('Recently Viewed Products') ?></span></strong>
    </div>
    <div class="block-content">
        <ol id="recently-viewed-items" class="mini-products-list">
        <?php foreach ($_products as $_item): ?>
            <li class="item">
                <a href="<?php echo $this->getProductUrl($_item) ?>">
                    <span class="product-image"><img src="<?php echo $this->helper('catalog/image')->init($_item, 'thumbnail')->resize(50, 50)->setWatermarkSize('30x10'); ?>" width="50" height="50" alt="<?php echo $this->escapeHtml($this->getProductName()) ?>" /></span>
                </a>
                <div class="product-details">
                    <p class="product-name">
                        <a href="<?php echo $this->getProductUrl($_item) ?>">
                            <?php echo $this->helper('catalog/output')->productAttribute($_item, $_item->getName() , 'name') ?>
                        </a>
                    </p>

                    <a href="<?php echo $this->helper('wishlist')->getAddUrl($_item) ?>" class="link-wishlist"><?php echo $this->__('Add to Wishlist') ?></a>

                    <button type="button" title="<?php echo $this->__('Add to Cart') ?>" class="button btn-cart" onclick="setLocation('<?php echo $this->helper('checkout/cart')->getAddUrl($_item) ?>')">
                         <span>
                             <span><?php echo $this->__('Add to Cart') ?></span>
                         </span>
                    </button>

                </div>
            </li>
        <?php endforeach; ?>
        </ol>
        <script type="text/javascript">decorateList('recently-viewed-items');</script>
    </div>
</div>
<?php endif; ?>

For replacing Description with Recently View products replace this code from catalog.xml

<block type="catalog/product_view_description" name="product.description" as="description" template="catalog/product/view/description.phtml">
    <action method="addToParentGroup"><group>detailed_info</group></action>
    <action method="setTitle" translate="value"><value>Description</value></action>
</block>

TO :

<block type="reports/product_viewed" name="product.description" as="description" template="reports/product_viewed.phtml">
    <action method="addToParentGroup"><group>detailed_info</group></action>
    <action method="setTitle" translate="value"><value>Description</value></action>
</block>

For Adding NEW TAB for Recently View products, Replace

<block type="catalog/product_view_description" name="product.description" as="description" template="catalog/product/view/description.phtml">
    <action method="addToParentGroup"><group>detailed_info</group></action>
    <action method="setTitle" translate="value"><value>Description</value></action>
</block>

WITH:

<block type="catalog/product_view_description" name="product.description" as="description" template="catalog/product/view/description.phtml">
    <action method="addToParentGroup"><group>detailed_info</group></action>
    <action method="setTitle" translate="value"><value>Description</value></action>
</block>
<block type="reports/product_viewed" name="product.recentview" as="recentview" template="reports/product_viewed.phtml">
    <action method="addToParentGroup"><group>detailed_info</group></action>
    <action method="setTitle" translate="value"><value>Recently View</value></action>
</block>