It would be the IP Address of the PC running MJ.
Under Win95, 98, or ME, you can get it by clicking on your Start Menu, then Run, then enter in "winipcfg".
Under WinNT, 2000, or XP you can get it by clicking on Start, then Run, then typing "cmd" and then in the DOS box, type "ipconfig".
If you wanted to try it from the same PC that MJ is running on, you can just go to
http://127.0.0.1
You have to start the server first. First, make you sure you have it started in Plugin Manager, then go to the Plug-ins branch of the Tree View, and choose "HTML Control Server". Then hit the Start Server button. If you want to change the port number it uses, you can, but then the address you connect to. If you changed the port number to 90, then the address would become
http://127.0.0.1:9