$ sudo apt-get install pptpd
The internal network IP address is 192.168.1.0/24, Ubuntu server IP is 192.168.1.2 and the Airport Extreme Router IP is 192.168.1.1.

$ sudo nano /etc/pptpd.conf
Add the following two lines,
localip 192.168.1.1
remoteip 192.168.1.230-235
(assigns 6 IP addresses for 6 VPN users)

$ sudo nano /etc/ppp/chap-secrets
Add one line,
vpnusername pptpd vpnpassword *

$ sudo nano /etc/rc.local
Add one line before exit0,
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

$ sudo nano /etc/sysctl.conf
uncomment
#net.ipv4.ip_forward=1

then, restart pptpd,
$ sudo /etc/init.d/pptpd restart

Configure router to forward TCP port 1723
The PPTP VPN server is behind a router(Airport Extreme in this case), configure the router to forward TCP port 1723 to the PPTP VPN server(192.168.1.2 in this case).

Using a Dynamic DNS service
Refer to:  Remote SSH Access to the Homeserver Behind A Router
As I use DynDNS.com in my case,  ddclient running on the server, I can connect the server by the address  myhomeserver.dnydns.com for example.