First, check that you have got a default route. By running netstat -rn (see netstat(1)), you should see two entries like this:
Destination Gateway Flags Refs Use Netif Expire default 10.0.0.2 UGSc 0 0 tun0 10.0.0.2 10.0.0.1 UH 0 0 tun0
This is assuming that you have used the addresses from the handbook, the man page or from the ppp.conf.sample file. If you do not have a default route, it may be because you are running an old version of ppp(8) that does not understand the word HISADDR in the ppp.conf file. If your version of ppp(8) is from before FreeBSD 2.2.5, change the
add 0 0 HISADDR
line to one saying
add 0 0 10.0.0.2
Another reason for the default route line being missing is that you have mistakenly set up a default router in your /etc/rc.conf (see rc.conf(5)) file (this file was called /etc/sysconfig prior to release 2.2.2), and you have omitted the line saying
from ppp.conf. If this is the case, go back to the Final system configuration section of the handbook.