Thanks. This is the follow-up, just in case somebody finds this information useful:
Following your suggestion I downloaded "EnterNet", but I didnīt like it very much because apparently the computer crashes if it goes to sleep without disconnecting pppoe first. EnterNet has a newer operation mode, that uses Appleīs `Remote Accessī, which supposedly solves the problem, but I couldnīt make it work. :o
But following hints from the help files I realized that there is another software for PPPoE called MacPoet
. This one is working well for me and apparently it is free.