Uhmm DNS servers go in the /etc/resolv.conf file
But your PPP should be pulling DHCP and adding those dynamically... BUT, if you can't ping by IP address - you've got other problems to worry about first.
Try doing an 'ifconfig' and see if you have an IP address assigned to your PPP config. If so, try pinging a regular IP address, that you KNOW will return a reply.. one that I know off hand that WILL return an echo-reply is: 184.108.40.206 .. which is www.yahoo.com.
Don't just try to ping anything, because most sites have echo-reply ICMP packets blocked. If you know the IP of your DNS server, try those as well.
If you can't ping anything, check your routes by typing 'route'. You should have a default route that points to a gateway, which on PPP I think is your own IP address. If you have no default route, then that is your problem.
Start there, and tell me what you get, and we'll work from there.