When Ubuntu 9.10 releases, pppoe connection via NetworkManager is impossible because some bug in it so to fix this we will install network manager from daily network manager trunk ppa.
Install network manager from Daily trunk PPA
First you need to edit the /etc/apt/sources.list file
gksudo gedit /etc/apt/sources.list
Addd the following two lines
deb http://ppa.launchpad.net/network-manager/trunk/ubuntu karmic main
deb-src http://ppa.launchpad.net/network-manager/trunk/ubuntu karmic main
Save and exit the file
Update the source list
sudo apt-get update
Install network manager
sudo apt-get install network-manager
Disable your previous “pppoe on boot” setting which is configured by pppoeconf.
Edit /etc/ppp/pppoe_on_boot file
gksudo gedit /etc/ppp/pppoe_on_boot
exec pppd call dsl-provider
#exec pppd call dsl-provider
Now you need to rename /etc/network/interfaces to backup file. NetworkManager will only handle connections which haven’t declared in interfaces.
$cp /etc/network/interfaces /etc/network/interfaces.back
Also edit /usr/share/polkit-1/actions/org.freedesktop.network-manager-settings.system.policy , find out the line contains “System policy prevents modification of system settings”, and below it there is a “auth_admin_keep“, change it to “yes“. This will enable you to edit a system wide connection. If you consider this will do harm to your security, then revert the change once you have set up your connection correctly.
Finally reboot your system
Source from here