Javascript input_box not displaying...

I have made the hacks, the templates are parsing, but the following code just isn't giving me the input_box popup it's supposed to (just like the PM alert that comes standard with vB):

Example code:

<script language="JavaScript">
<!--
function confirm_entry()
{
input_box=confirm("Click OK or Cancel to Continue");
if (input_box==true)

{ 
// Output when OK is clicked
alert ("You clicked OK"); 
}

else
{
// Output when Cancel is clicked
alert ("You clicked cancel");
}

}
-->
</script>

Should this be within the header tags, or is withing Body tags ok? (I've tried both, tho...)

Is there another snippet of javascript I need to get it running?

Can anyone help?

Thanks
Mark