Does Interix implement fork()?
The page you're looking at is the *nix to Windows porting guide. It doesn't show you how to use fork() but the closest win32 equivialent, CreateProcess. The pages there documents which Win32 function you should use instead of Unix functions.
You'll need the subsystem for Unix and the SUA SDK to use fork(). There you'll get a *nix environment on Windows, fork() will be in the usual unistd.h library, and you'll link to libc.so (using gcc) to use it.