The new default for the PPP login script is /usr/lib/linuxconf/lib/ppplogin, which is supplied. /etc/ppp/ppplogin was a suggested value before the pppdialin module became available. Newer linuxconfs default to this path.
To change the default ppp login script, you go in the user account/policies menu and pick "Available PPP shells". Creating a suitable PPP login script requires some understanding of the pppd utility though. Using the pppdialin module will help. You may want to check the supplied ppplogin script anyway.
Once you have enabled the pppdialin module, you will see a new entry in the user accounts/policies menu. Select it and read the help screen
Note that you must also enable the serial port. You do this with the mgetty+sendfax package (included in most distribution). You may want to install the mgettyconf module found at ftp://ftp.solucorp.qc.ca/pub/linuxconf/modules to help configure that.
Once done, you create PPP accounts.