Assuming your iframe has an ID of "myiframe", try altering one of the menu items so it reads:
linkset[0]='<div class="menuitems"><a href="javascript:document.getElementById('myiframe').src='http://www.dynamicdrive.com'">Dynamic Drive</a></div>'
In other words, the code:
javascript:document.getElementById('myiframe').src='http://www.dynamicdrive.com'
should be used as the href portion of the menu link.