Click on the subject of the spam in your inbox or other folder. This displays the message.
At the top of the message you will see the following links in the message frame right above the "reply" buttons:
[folder name] : Prev | Next : Download
Select "Download" from the above.
This changes the message window into the download window. In most cases you will see a line like this:
Full message: Content-Type: message/rfc822, with "Full Message" as a link.
(Note, depending on the spam, the "Content-Type" may not match the sample above. This shouldn't matter...)
A new browser window will spawn with both the headers and the message text. At this point, simply copy all the text in order to paste it to the form or in a message to whomever, should you be doing your own spam fighting :)
You may then close that full message browser window. To get back to the folder where the spam was, simply click on the [folder name] link either in the message window, or in the onebox.com navigation frame to the left.