You just need to change the target's name...
Take this example element.
<input type="CHECKBOX" name="CHECKBOX_1" value="This...">
In this case the target name is CHECKBOX_1 which is then reflected in the Javascript.
You just need to find out the name (name="example") for your checkbox and update the Javascript to reflect that.
You'll need to place the Javascript in between the <head> tags in your header.php
Here's an example (you'd need to change where i've put "NAME-HERE")
<script type="text/javascript" language="JavaScript">
<!--
function checkCheckBoxes(theForm) {
if (
theForm.NAME-HERE.checked == false)
{
alert ('You didn\'t choose any of the checkboxes!');
return false;
} else {
return true;
}
}
//-->
</script>
And also add onsubmit="return checkCheckBoxes(this);" onto the comments form.. (comments.php in your theme - same file you've proberly added your checkbox to).
It's untested, but it should be that easy (i can't see why not)....
:)