Cisco PIX Firewalls require two elements to pass traffic from outside (higher security) to inside (lower security): a static translation and a conduit.
For this example, assume a server has IP address 192.168.1.100 and there is an available outside address of 126.96.36.199.
First, create the static translation. This configuration line establishes a relationship between 188.8.131.52 (public Internet IP address) and 192.168.1.100 (inside, private IP address).
fixup protocol pptp 1723
static (inside,outside) 184.108.40.206 192.168.1.100 netmask 255.255.255.255 0 0
Next, create appropriate conduits to allow specific traffic to pass from the outside to the Inside interface. PPTP uses TCP/1723, and IP/47 GRE.
conduit permit tcp 220.127.116.11 eq 1723 any
conduit permit gre host 18.104.22.168 anyor
access-list 101 permit tcp any host 22.214.171.124 1723
access-list 101 permit gre any host 126.96.36.199
access-group 101 in interface outside
A couple of notes:
In the conduits and access-lists, the any keyword allows matching traffic from any IP address to pass through the firewall. This should be replaced with the source IP address of the PPTP tunnel, if at all possible.
In the access-lists, verify any existing access-lists or other traffic needed before entering the last line!