I had the same problem but found the driver for Mac OS > 10.5.1 (Leopard) (it works also for 10.5.7 my version) in "OTENET" (the greek national telecommunication company) and you can download it here: http://www.otenet.gr/hd/downloads/SA..._v406_Intl.zip
Before the installation of the driver go to 'System Preferences'>'Network" and create a new location (in my case I called it 'ADSL USB PPPoE / PPPoA' a name suggested by the provider 'OTENET'). If you do not create a new location there will be serious conflict and MAC will need constantly to restart it.
After the creation of a new location run the driver, plug the USB when you prompt by the installation process and restart your computer after the installation as you will be prompt by the driver's installation. Apply the changes to 'Network'. You will find the creation of a new connection called 'Etnernet...or (en2) self-assigned IP'. That's good news.
You have to add now a new PPPoE connection to this new location (you can add connection using the symbol '+' in the bar below the connections).
After the creation of the new PPPoE connection, choose it and add your user name and password of your internet account, press connect and that's it.
Hopefully you will connect...
keep in mind that when you 'll create the new PPPoE connection turn it to the 'Ethern...or (en2)' choice you have from the menu it opens immediately after the creation of the new PPPoE connection.
This solution is not very stable... I had problems when my mac updated to OS X 10.5.8 and obliged to return to 10.5.7. But 10.5.7 also is not very stable... sometimes it needs to reload the driver and do everything again from the start in order to connect to internet.
Thank you so much for your responce.I will try it just because i wanna see if i can "make" it work. I was waiting several months for a message so i bought a new modem for mac..
Thank you a lot for your answer.
With regards "pattousa"