FSO == FSSCP, the two names are used interchangeably. FSO stands for FreeSpace Open, which is the exe produced by the FSSCP (FreeSpace Source Code Project). Installing the FSO goodies is rather easy, you can use either the automated Installer found
here, or the manual guide found
here.
Note that our main mirror for downloads, freespacemods.net, is ATM down due to a malware infection. If you have trouble locating a file, post here, and we'll see to it that you'll get set up.
If you should run into issues after installing stuff,
this FAQ should be your first port of call. Alternatively, the hard-light IRC channel (link is in my sig) is always open for tech support business.