After you get to the page that takes you to, click FLHook.dll and then on the page that comes up, click Download. Might not have to download the FLHook.ini file as well, but I always do any time I update FLHook, just to be safe.
Once you have these files, drop them into the EXE folder of the server's freelancer installation directory replacing the files that are already there with the same name. Make sure all the settings are correct for your server in the FLHook.ini file, save and exit it and you're good to go.
Edit: You'll need to restart the server or rehash for the changes to take effect. Might need to do a server restart as I don't know if Rehash reloads flhook.dll. I know it reloads the others; don't know if it does the same for FLHook.
Should be the My Documents\My Games\Freelancer\Accts\MultiPlayer
I have found that Version 1.4 and 1.41 seem to have a problem on the first run of the program. They can't find the player database even when it's pointed to the correct folder. At least it was that way on mine. I corrected it by downloading DSAccountManager V1.1. I ran that program, let it create the player database and then closed it. 1.4 worked flawlessly after I did that. I get an error every time I run 1.41.