Port forwarding won't help since you can't connect to a server not a server cannot connect to you. Port forwarding only enables a port to work if you're running a server on your own machine.
Default port for minecraft is 25565 so you can connect fine to servers on that port. More likely than not you are entering the IP address format wrong or the program is not recognizing it. Also read the port forwarding warning on the Minecraft wiki pages:
http://www.minecraftwiki.net/wiki/Setting_up_a_server