To obtain the address of your FileMaker Server computer follow the steps below. Please note these steps must be followed on the actual server itself not on your desktop.
Instructions for Windows
Click on Start -> Run -> type in "cmd" (without quotes) in the box and press Enter.
This will open the command prompt. Type "ipconfig" (without quotes) and press Enter.
This will display your ip configuration.
Some versions of windows may access the command prompt differently. You can get more information here Microsoft Command Prompt
Instructions for Mac OSX
From the Apple menu pull down "System Preferences"
Click on the "Network" preference pane
Your IP address will be visible to the right, as indicated in the screenshot below.