I think you simply need to reinvent the image of the site, as I stated above. Agreeing with the others, the site is good, but being as people don't like registering to begin with, I assume you could settle on a very simple download / upload site. Most people who make fan sites and upload sites normally fail because they try to become a new "keystone" site. Personally, I think an option to download without account should be offered in order to keep a quick interest in the site. All too many times people require registration, and the registration systems are broken, or it's for a site that offers little in downloadable content to suffice for the inconvenience of registering.
The next thing you need to do (note that these are just market studies I've done for a project based on common failures in startup websites), is advertise a bit. Just saying about it in passing isn't enough to peak an interest. Put advertisements in your signature, and any time people are looking for places to upload custom content, offer your site. There is nothing wrong with a bit of shameless advertising. I'm not suggesting you go and make a thread every other day, simply keep people interested.
Look at hosting sites like Rapidshare and Megaupload. These are key examples of crappy sites that get a lot of notoriety due to their convenience and quickness. Sure, you have to wait annoying times, but people will do anything to host files quickly and easily, no matter what other people are doing. I suggest a system of allowing open uploads to registered members, meaning anyone can upload their packs. Sure, it'd be more responsibility on your part, but it'd keep things flowing and give people a nice place to host their content.
Next thing I'd suggest would be to choose a simple, clean, and non-cluttered design. No offense, but your enter screen is a bit annoying for someone who wants to grab a quick file, and your design is a bit cluttered being as you use a pre-made CMS (not theme, but CMS, which appears to be PHP-Fusion, correct me if I'm wrong). Sites that are simple to navigate are ones people like. Look at Google, a simple search box. Nothing real major. Straightforward links, etc. Your best bet is to code yourself, though if you have a lack of time use a CMS like Joomla which can be customized to the very pin point settings.
The main problem with lack of popularity is the lack of people knowing about it, as stated by others. Old members know what it is, but the new generation of interested game creators have no clue what it is. Take xplosys site as an example. It was simple, and due to that simplicity it was a fairly popular site.
I don't want to sound arrogant, but for 5 years I went down this road trying to get a startup site going, and even then it was troublesome because it's hard getting a site popular and offer people things. What I find works is to keep things simple. Keep things simple, easy, and direct. Don't add features that you don't really need such as news feeds relating to FPS Creator. People can see that stuff on the forums, thus no need to elaborate. If it was news about the site, sure, but otherwise I wouldn't take the time. Same with forums. People don't like talking on forums that are inactive.
Wow. Quite a bit typed, sorry, I tend to go wild when typing.
I just now realized I've had a typo in my signature for the past 3 years.
