Someone I know has kindly written up a script that does exactly what I need, even using my page source so it should work out the box.

He sent me this link to show me it's working, and said to copy the JS from this. http://jsfiddle.net/CPjpt/2/

I put the code into a file, and saved as jscript_formdim.js. I then put this file in /public_html/includes/modules/pages/no_account as no_account is the name of the page I need this script to work on.

If you go to my checkout page, you can see that the the script has been loaded, however the form does not dim when checked.
I did notice in my dev tools console this error: Uncaught SyntaxError: Unexpected token ?

Can someone please tell me what the means, do I need to place the code in a specific location or on a specific page?