Because your using 6.4, there's no way to alert users outside the PM Folders, such as the forum summary etc... Unless that's what you want to do?
To do it in the PM Folders, here's what to do:
open ubb_folders.cgi
[code][/code]If your wanting it on the forum summary, then you'll need to install the
PM Notification Hack(click here for it) . And then you can modify it exactly as i mentioned above with the javascripts.