How do I indicate that a checkbox should be checked?

I have a form named "FrontPage_Form2" and a checkbox named "checkboxPowerUnit"; and, I want to programmatically change whether or not it is checked by default based on a selection made in a selection box. I have tried:

and every one results in an error saying "document.whatever" is null or not an object. Can anyone tell me how to indicate that this checkbox should be checked or unchecked?

re: How do I indicate that a checkbox should be checked?

Assign it an id and then use document.getElementById(idstring)

re: How do I indicate that a checkbox should be checked?

it has an id. the id and the name are the same.

re: How do I indicate that a checkbox should be checked?

also do not work

re: How do I indicate that a checkbox should be checked?

Are you perhaps running this script before the page has loaded?
if document.getElementById('checkboxPowerUnit') is giving you a null object, then that object does not exist

http://www.w3schools.com/JS/tryit.as..._input_checked