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.