how to configure pptp and freeradius

pptp authentication using freeradius you can following tutorials.

For freeradius setup : http://safesrv.net/install-and-setup-freeradius-on-centos-5/
For pptpd setup      : http://safesrv.net/setup-pptp-and-freeradius-on-centos-5/

After radius server installation test radius server locally and remote server. If response is positive then you are done with radius server. Else add nas table to radius server if is it is already not included or not already configured.

Try not to jump on another server deployment util you are not completely deployed previous one.

If you are unable to communicate between user , VPN server and freeRADIUS server then check IPTABLES and port forwarding. And also check that in VPN server configuration ,  radius server ip is defined.
Also confirm that in freeRADIUS configuaration ,   VPN server ip is listed in clients.
After successfull setup of these two servers following are possible issues you may face.

Possible issue :
Issue 1 : Carefully read tutorial and do configuration of right files. Because some directories contains file names with little difference for example options.pptpd and options-pptpd.

Issue 2 : User requires some authentications like pap , chap , mschap etc. User sends authentication requests to VPN server and VPN sever forward this request to freeRADIUS server to authenticate user.

Authentication will only be successfull if user's required authentications matches with /etc/ppp/pptpd-options configuration. In /etc/ppp/options-pptpd  authentications are written like this require-pap refuse-pap etc.

Still having problem please read this freeradius official tutorial http://wiki.freeradius.org/guide/PopTop-HOWTO   .