1/21/10 - I've worked out the bug with PM links not triggering the popup, and confirmed compatibility with the five major browsers. If you've previously downloaded Popup PM, you'll want to redownload it, reupload the uploaded files, uninstall the old product and install the new one.

Hope you enjoy!
Settings Quick Tutorial
All options for the Popup PM are found in Admin CP > vBulletin Options > Popup Private Messaging (located under Private Messaging Options).
To change the popup window size, change the options for window width and height. Enter a number in pixels (number only).
To specify which usergroups the popup is enabled for, change the option for usergroup permissions. Enter the usergroup IDs of the groups to enable the popup for. Enter the numbers separated by commas, i.e. 2,5,6,7 - you can also leave the field blank to enable for all usergroups, and set to a non-real usergroup ID (like 1000) to disable the popups for everyone. Note that even if you give unregistered users or users without private messaging permissions access to the popup PM system, it won't grant them private messaging permissions. They will see a "no permission" error
within the popup window.
Quick Troubleshooting
Literally, the only thing you can really mess up during installation is not properly uploading the file(s) in the UPLOAD folder. In other words, make sure rw_popuppm_free.js gets into your
www.mywebsite.com/myforumdirectory/clientscript/ folder. Then install the product XML file in your Product Manager. Done.
If you're having issues with the popups not showing up, check for script errors in your browser. They can tell you what's not working and why. Chances are that something else installed on your forum is interfering with the popup PM script. Also check to make sure you didn't set the popup window height/width to 0 or something for some silly reason.
If you use a custom style or have customized your templates, this seems to be the other thing that breaks the mod. Look at your
footer template and find:
HTML Code:
<script type="text/javascript">
<!--
// Main vBulletin Javascript Initialization
vBulletin_init();
//-->
</script>
Below it, you should have:
HTML Code:
<script type="text/javascript">
<!--
// Main vBulletin Javascript Initialization
vBulletin_init();
//-->
</script>
{vb:raw template_hook.footer_javascript}
The footer_javascript template hook was added in vBulletin 4.0, and it seems some users with custom styles are either accidentally removing it or copying/pasting a lot of code from their old styles which didn't have the template hook. Add that and you should be good to go.