I'm not sure if anyone has PM'd you, but since there aren't any stickies about this, I can give you some quick instructions (which are good for this incarnation of the board system vBulletin Version 3.0.0 Release Candidate 4).
You might want to save your MP3 into "My Documents" or "My Music" folders for easy access.
2. When you create a new post on this board, you should see a text entry box and a section called "Additional Options" below the "submit reply" button.
Scroll down and you should see the "Manage Attachments" button.
3. Click on "Manage Attachments", and a new window should open.
4. Click on "Browse"
and find the folder you saved your MP3 in, select your MP3 and click "Open" to link the filename into the window.
5. Click on "Upload".
The window should do some processing.
6. Click on "Close Window". The window should close and you should see something similar to the image below.
This site has some pretty hefty restrictions on the file-type posted (to prevent hijacking and weird files) so make sure your MP3 recorder and the resulting file is legit. Windows Media Player comes with a builtin MP3 recorder. It records only up to radio quality, which should be more than adequate for promotion.