Port Forwarding on Telkom 5100 ADSL Router

Hi all
I need to know how to forward ports on Telkom DSL 5100 Router.I know it is in the Nat section and edit SUA details.I can only forward port 3389 to one ip address.Also if i use any other port other than 3389 to an internal ip address it does not work.i have setup one port forward on 3389 to an internal ip address but would like to make a few other pc’s remotely accessible but am stuck as i cannot forward port 3389 to another internal ip as it says the port is already used.Any help will be greatly appreciated.Thanks!