Turn off the Windows Firewall, and any extra firewall your security software might have added. If it starts working, turn the firewall back on, and adjust firewall settings appropriately.
The URL is the form of: http://xxx.xxx.xxx.xxx:5481 with the IP address (not localhost, and not 192.168.0.0) of the 3CX substituted in.
I can say for sure that it works with Cassini on machines other than that which it is installed on. Working with it that way every day.
After initially posting, it occured to me that your use of "192.168.0.0" MIGHT not have been as a bogus example address, but something you were actually attempting to work with. So I Googled it, just to have something to copy/paste and confirm I wasn't nuts. From About.com:
.About 192.168.0.0
Wednesday April 9, 2008
| Definition | The IP address 192.168.0.0 is the start of the Class C private range. Broadband routers and other network gateways reserve this address to reference the private network. You should not attempt to set 192.168.0.0 as the IP address for your router or any computer as it will not function properly. Some routers are configured instead to use 192.168.0.1 by default that is the first usable address in the range.
In other words, don't ever set any equipment on your network to end in ".0"