can''t get ftp working...

Hi

I have a speedtouch Pro modem/router and connected to that (via a
switch) a Asus wl500g deluxe wireless accesspoint.
Attached to this accesspoint I have connected a USB disk which can be
accessed through ftp. Local access works fine, but I can't get it
through my speedtouch Pro to be able to access it from the outside
world. I have mapped port 21 to my accesspoint's WAN Ip address, but
when I try to contact it I keep on getting the message 'getting
directory contents' (access through IE)
What else do I need to configure in my speedtouch modem/router?

Frank