Just so we're clear..
it should be
rel="thickbox_detailed" not rel="[thickbox_detailed]" right?
Here's the entire snippet.
Code:
{if $product.tmbn_url}
<a href="{$product.tmbn_url}" title="{$product.product|escape}" class="thickbox"
rel="thickbox_detailed">{include file="product_thumbnail.tpl" productid=$product.productid image_x=$config.Appearance.product_image_width product=$product.product tmbn_url=$product.tmbn_url id="product_thumbnail" alt=$product.product|escape}
{if $product.tmbn_url}<div align="center" class="SmallText">+ Click to Enlarge</div></a>{/if}
{/if}