It would be nice to include built in support for PPPoE in the next version of LinuxMCE. To get it working on my machine, i had to disable the built in firewall and execute the following to get internet sharing to work:
iptables -t nat -A POSTROUTING -j MASQUERADE
Hindsight tells me i could of avoided a lot of pain by just modifying the device in the database like the wiki says, but it shouldn't be this hard to set up something so simple.
It would be nice, but I have only seen a few people encountering the same problems we faced.
I had the same problem with PPPoE and had to do the manual configuration. I also filed a bug report in Mantis on http://www.linuxmce.com/mantis/view.php?id=3558, but it seems to be low on the developer's priorities.