OK, so what you need is Network > Router > Virtual Servers, not Firewall.
You need to add rules in there. The step by step requires more info, like what interface is connected to what. So for example for VNC:
1. Set Input Interface to the ethernet port connected to the ADSL modem, or PPP link, depending on your configuration.
2. Set IP Address to ANY.
3. Set Protocol to TCP.
4. Set Local Port to 5901
5. Set Remote IP to 184.108.40.206
6. Set Remote Port to 5901