I am trying to modify a form element using hook form alter but I can't find the correct field name
When you can't find an array key, the best thing to do is to dump the entire form array and search for it. If you use the Devel module, you can use the following:

dpm($devel);


This will give you a nice collapsible output through which you can search. Or you can use the old-school method that I use, and dump it directly using:

die('&lt;pre&gt;' . print_r($form, TRUE) . '&lt;/pre&gt;');