sono proprio una schiappa con JS! ci deve essere qualcosa che non va...
Con il codice originale mi apre la finestra (quindi il meccanismo dovrebbe funzionare), ma se lo sostituisco e provo a farlo puntare sull'iframe sotto non succede nulla...
se hai un attimo ti posto tutto il file html, basta che copi un un file vuoto e salvi...

Grazie dell'aiuto sarei nei guai senza di voi!!!
^_^

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1" />
<title>Documento senza titolo</title>
</head>

<body>
<form name="dynamiccombo">
<select name="stage2" size="1" onChange="displaysub()">
<option value="#">This is a place Holder text </option>
<option value="#">This is a Place Holder text </option>
<option value="#">This is a Place Holder text </option>
<option value="#">This is a Place Holder text </option>
</select>
<input type="button" name="test" value="Go!"
onClick="gothere()">
</form>

<script>
<!--

//2-level combo box script- by javascriptkit.com
//Visit JavaScript Kit (http://javascriptkit.com) for script
//Credit must stay intact for use

//STEP 1 of 2: DEFINE the main category links below
//EXTEND array as needed following the laid out structure
//BE sure to preserve the first line, as it's used to display main title

var category=new Array()
category[0]=new Option("SELEZIONA LA REGIONE", "") //THIS LINE RESERVED TO CONTAIN COMBO TITLE
category[1]=new Option("Lombardia", "combo1")
category[2]=new Option("Sicilia", "combo2")
category[3]=new Option("Lazio", "combo3")

//STEP 2 of 2: DEFINE the sub category links below
//EXTEND array as needed following the laid out structure
//BE sure to preserve the LAST line, as it's used to display submain title

var combo1=new Array()
combo1[0]=new Option("Milano","http://www.whymambo.com/index.php?option=com_content&task=view&id=94&Itemi d=105")
combo1[1]=new Option("Bergamo","http://www.whymambo.com/index.php?option=com_content&task=view&id=95&Itemi d=104")
combo1[2]=new Option("Como","http://www.whymambo.com/index.php?option=com_content&task=view&id=93&Itemi d=106")
combo1[3]=new Option("TORNA ALLE REGIONI","category") //THIS LINE RESERVED TO CONTAIN COMBO SUBTITLE

var combo2=new Array()
combo2[0]=new Option("Palermo","http://www.whymambo.com/index.php?option=com_content&task=view&id=92&Itemi d=110")
combo2[1]=new Option("Messina","http://www.whymambo.com/index.php?option=com_content&task=view&id=91&Itemi d=111")
combo2[2]=new Option("Catania","http://www.whymambo.com/index.php?option=com_content&task=view&id=90&Itemi d=112")
combo2[3]=new Option("TORNA ALLE REGIONI","category") //THIS LINE RESERVED TO CONTAIN COMBO SUBTITLE

var combo3=new Array()
combo3[0]=new Option("Roma","http://www.whymambo.com/index.php?option=com_content&task=view&id=89&Itemi d=108")
combo3[1]=new Option("Latina","http://www.whymambo.com/index.php?option=com_content&task=view&id=88&Itemi d=109")
combo3[2]=new Option("Frosinone","http://www.whymambo.com/index.php?option=com_content&task=view&id=80&Itemi d=107")
combo3[3]=new Option("TORNA ALLE REGIONI","category") //THIS LINE RESERVED TO CONTAIN COMBO SUBTITLE

var curlevel=1
var cacheobj=document.dynamiccombo.stage2

function populate(x){
for (m=cacheobj.options.length-1;m>0;m--)
cacheobj.options[m]=null
selectedarray=eval(x)
for (i=0;i<selectedarray.length;i++)
cacheobj.options[i]=new Option(selectedarray[i].text,selectedarray[i].value)
cacheobj.options[0].selected=true

}

function displaysub(){
if (curlevel==1){
populate(cacheobj.options[cacheobj.selectedIndex].value)
curlevel=2
}
else
gothere()
}


function gothere(){
if (curlevel==2){
if (cacheobj.selectedIndex==cacheobj.options.length-1){
curlevel=1
populate(category)
}
else
location=cacheobj.options[cacheobj.selectedIndex].value
}
}

//SHOW categories by default
populate(category)

//-->
</script>


<iframe name="destra" width="100%" height="400px" scrolling="no" src="http://www.google.com" >
</iframe>

</body>
</html>