Having an open firewall port on the Mac would be a moot point if the Mac is the VNC client, as it initiates the outgoing connection - it's not running the server.
Now on the Amiga, if you have a firewall (is there one?), then you'd better open the port(s) if the firewall shuts down incoming connections by default (why wouldn't it!??! isn't that the point?)
Anyhow, I don't have Chicken Of The VNC in front of me, but I recall on the PC using TightVNC, when you specify the port, you *must* use two colons (

between the IP and the port. Maybe give that a try?
Also, I'd recommend hard-coding the port initially (default is 5900 if I recall correctly), just to eliminate any unneeded frustration.
Good luck moto