Thank you for your post.
If you are using a local network, then make sure you are using the local IP address. It will probably start with 192.168.xxx.xxx. One way to get the local address is to go into System Preferences and select "Network". Click on AirPort and on the right side, it will tell you if AirPort is turned on or off. When on, there will be some information just below showing the local IP address.
If you need to turn on port forwarding, go to www.portforwarding.com for help configuring port 5003.
For more information, see Knowledge Base Article #7768:
Let me know if you still need additional help.
Thanks for the replyTSGal.
After much tearing of hair and gnashing of teeth I got it work on the LAN
For anyone reading this post later, I also followed the instructions on http://homepage.mac.com/car1son/static_port_fwd_intro.html which seemed a bit more specific for my situation regarding setting up the Mac.
Still no joy when trying to access through the 3G network though (I turned off the wifi on the phone and entered the IP I got from whatsmyip.org/).