my search form is a little different. Maybe it will help?
PHP Code:
<form action="{$my_pligg_base}" method="get" name="thisform" id="thisform">
<label for="searchsite">Search:</label>
<input type="text" size="20" name="search" id="searchsite" value="{$searchedfor}"/>
<input type="submit" value="Go" class="f-submit" />
</form>