How do I configure my router to allow incoming port 88 connections?

I am running MAMP on my Mac OSX 10.8 using a dynamic DNS.

If I bypass the router I can access the web server from the outside using the url from my dynamic DNS.

If I type http://127.0.0.1:8888/ locally I can see the web server.

I have a Netgear WNDR3700 with the Dynamic DNS setup and "Show Status" shows it is working.

I also have Port Forwarding turned on with the machine for port 88.

However, when I try to access the web server it says:

Unable to connect
Firefox can't establish a connection to the server at mydomain.com:8888.
The site could be temporarily unavailable or too busy. Try again in a few moments.
If you are unable to load any pages, check your computer's network connection.
If your computer or network is protected by a firewall or proxy, make sure that Firefox is permitted to access the Web.

Any help would be appreicated.