1. Make sure your OSX firewall is turned off. Else configure it to allow LW; OSX Firewall settings for Tiger

2. Try deleting your LW preferences folder whilst the program is CLOSED. The prefs folder is found here: /Users/your_username/Library/Preferences/LimeWire

Often when changing over to LW pro, it's a good idea to delete the LW prefs folder.
If these 2 things don't help, let me know. Perhaps a different version will help. That's if the problem is not your ISP.