TopPop3 is not showing up in the Growl application list on either computer.
Try downloading the updated version.
Let me know if it works.
If it doesn't: Please download the attached file. Unzip it and mount it.
On the disk image you'll find "growl.framework".
Copy that to either
If you don't know what I'm talking about then create a folder and move TopPop3 or iMapper to it and place the framework into that folder too.