I just installed Private Messaging and having some issues with the plugin on a MacBook with the latest versions of WordPress and BuddyPress installed.
When opening the Send New Message window, the window is not displayed correctly. Please see the attachment (screen shot). I disabled all extensions of Safari, tried it with Firefox and Google Chrome, but all same.
Did you tested the plugin on an Apple MacBook? Maybe is a Java problem.
Thanks for your feedback, and regards,