OK,
Go to the Airport Utility
Select your router and choose manual setup
Select Internet from the top tabs and then DHCP
In DHCP reservations enter the MAC address of your LinkStation to reserve that address for it
Next
Click NAT from the top tabs and then configure port mappings (make sure the enable port mapping protocol is checked)
for all public and private ports enter 9000
for the private IP address enter the IP address of your LinkStation.
This is how I got it to work, can't say if it's the best way or even a good way to do it.
Good Luck