The FreeSpace Port needs to be installed in a folder called
fsport, not
FSport_one. A number of mods require it to be installed there. Also, FSPort 3.1.1 is old; the latest version is 3.4, which can be found here:
http://www.hard-light.net/forums/index.php?topic=84358.0. Make sure you delete your current FSPort installation first.
Actually, I suggest using the FreeSpace Open Installer, which you can get here:
http://www.fsoinstaller.com/. (No, this is not the same installer that everyone was told to avoid like the plague a few years ago.) In addition to the main files, the installer can install a number of mods, including FSPort, and remove old files in the process.
If you ever need to install a mod manually, remember to read any installation instructions that come with that mod or that are in its release thread (if there is one). If a folder name is suggested, use it. If a mod is packaged with its files already in a folder other than "data," use that folder name. Otherwise, you can normally use any name you want.
And yes, you select a mod by selecting its image in the Mods tab, but you also need to click "Activate." That last step is important.