To add a new port mapping click on the add button to open the Add Port Mapping dialog.

Name - Give the port mapping a name.
Local IP - The IP address to forward the network traffic to.  You can select from the drop-down list of local addresses or type one in yourself.
Internal Port - The internal port to forward the network traffic to.
External Port - The external port to forward the network traffic to.
Protocol - Select a network protocol either TCP or UDP.

Press the Ok button to add the new port mapping.