I am able to connect to a m0n0wall v.1.11 PPTP server using pptpclient 
running on FreeBSD 5.3, but I cannot interact with any machines behind 
the m0n0wall.

   FreeBSD client <--> Internet <--> m0n0wall <--> target computer
              ^---can connect to here --^            ^
              ^---cannot connect to here ------------^

Here is my ppp.conf

  set authname xxxxxxx
  set authkey xxxxxxxxxx
  set timeout 0
  set ifaddr 0 0
  add xxx.xxx.xxx.xxx/16 HISADDR
  alias enable yes

This config works to connect to a Windows PPTP server.  Do I need 
something different to connect to a m0n0wall PPTP server?

(BTW, I can connect through the m0n0wall with a Windows PPTP client.)

Thanks,

Scott