If you're having trouble accessing WeatherCat from a client over the internet, first, check your current Internet IP address and try that in the client instead of the dyndns domain name - if that works then the problem is dyndns do not have your current IP address. you can find your current IP address at: http://www.whatsmyip.org/
Secondly, check the ports you have forwarded are forwarded to the right machine on your network (i.e. the machine running WeatherCat). You can do this by going to that machine, then System Preferences->Network - the machine's IP address will be displayed there. Also check that forwarding is actually enabled - there may be something in routers configuration that is preventing forwarding.
Third, check if you have the firewall enabled on the machine running WeatherCat (System Preferences->Security & Privacy and select the Firewall tab. Check the settings there (I think by default this is off and allows all connections).
Forth, check if you have any third party firewall/security software running - you may need to inform that about the port you wish to open.
Finally, make sure Peer to Peer Applications are not blocked.