I think you are confusing the "from" address with the "reply to" address.
In Mac Mail the "from" address field is used to select the email account that you wish to use to send the the message. It is a drop down list box for selecting the account.
The "reply to" field is a text box that you can type anything into (like in your linked article). If the "reply to" field is not visible in the message edit window, you can add it with the View > Reply to Address Field menu item.
lol - so my wording wasn't so clear and I didn't mean to sound THAT dumb - no I'm not confusing the "from" address with the 'reply-to' - I know the difference.
In MS Outlook Express, you can supply your email address for the account to access the mail server, but you can also specify a 'reply-to' address for all out-going email as seen in this screen-shot for Outlook Express.
Just wondering if Mac Mail had the ssame type setting, which it doesn't appear to.