skin/common_files/customer/search.tpl
After...
Code:
<span class="search">{$lng.lbl_search}:</span>
Insert...
Code:
<select name="posted_data[categoryid]">
<option value="">{$lng.lbl_all}</option>
{foreach from=$categories_menu_list item=c}
<option value="{$c.categoryid}">{$c.category}</option>
{/foreach}
</select>