Kernel version 2.6.15 or above has MPPE built-in which is required for MSCHAPv2. Fedora Core 5 and 6 kernel version on the CD are 2.6.15 and 2.6.18 respectively. That means you do not need to install the MPPE module as you have to for Fedora Core 4. Information for FC4 and MPPE is in Appendix A1 and A1.1.
To test if your kernel supports MPPE:
[root@pptp ~]# modprobe ppp-compress-18 && echo ok
It should resturn an "ok".
Note: The latest kernel for Fedora Core 5/6 at the time of writing is 2.6.18.
FC5 comes with ppp-2.4.3-6.2.1 and FC6 has ppp-2.4.4-1. MPPE support is already enabled. If ppp is not installed, use yum to install it.
[root@pptp ~]# yum install ppp
Note: Information on how to update ppp module on FC4 is in Appendix A2.
From here download the pptpd rpm, pptpd-1.3.3-1.fc5.i386.rpm or pptpd-1.3.3-1.fc6.i386.rpm according to your Fedora version, and install it.
[root@pptp ~]# rpm -ivh pptpd-1.3.3-1.*.i386.rpm
Note:
The pptpd FC6 package is built against ppp-2.4.3 but FC6 has ppp-2.4.4. It prevents the tunnel to be established and gives the following error message is in /var/log/message.
Plugin /usr/lib/pptpd/pptpd-logwtmp.so is for pppd version 2.4.3, this is 2.4.4
The workaround is to disable the logwtmp option in /etc/pptpd.conf.
I have produced a patched RPM to address the problem. You may find a copy from here.