You might note the URL you are using.
There is the IP Address you are given by your ISP - an internet URL (http://whatismyIPAddress.com)
This IP Address is visible from the internet and ends at your DSL or Cable Modem / Router. (http://en.wikipedia.org/wiki/IP_address)
Your Local IP Address (http://en.wikipedia.org/wiki/Private_network) is most often a 192.168.xxx.xxx or 10.0.x.xxx.
This is visible only to the computers sharing your network.
Requests for things you are sharing have ports - webpages most often are on Port 80 - FTP requests Port 21 - FMP Port 591
You may need to forward requests for FMP access from your router to the computer hosting FMP (http://en.wikipedia.org/wiki/Port_forwarding)
Instructions for this can be found here http://portforward.com
FROM FILEMAKER HELP
The TCP/IP address in the Instant Web Publishing dialog box determines where web browsers can find FileMaker Pro databases on the host computer. By default, FileMaker files are shared from port 80. If port 80 is already in use (by a web server or another application), consider changing TCP/IP Port Number to 591. FileMaker, Inc. has registered port number 591 with the Internet Assigned Numbers Authority (IANA) specifically for web publishing.
Google "iwp site:forums.filemaker.com"