<ul id="idframe" style="display: block"><iframe name="tree3" height="200" width="748" scrolling="no" frameborder="1"></iframe></ul>
<a href="#" onclick="obj=document.getElementById('idframe').style; tmp=(obj.display!='block') ? 'block' : 'none'; obj.display=tmp; return false">Cкрыть / Показать</a>