Looking for live help? Then join us for an upcoming webinar to chat with staff and other Wikia editors. You can also check our Staff blog to keep up-to-date with the latest news and events around Wikia. Lastly, check out the Wikia Video Library, where you can find premium licensed videos to add to the wiki.
All of these links are a great way to start exploring Wikia.
You're right, it looks like none of them have been around for several months. That means this wikia is eligible for adoption. If you think you might be interested in editing here long-term, and want to look into adopting it, here's where you can find out more: Help:Adoption
I've given it a try, and was able to add the category without a problem. If you're sure you've pressed Enter after typing in the category, and the "Save" button still doesn't activate, the problem probably lies with your local browser settings.
You could try using a different browser, or delete cache and cookies, update, and restart your current browser. That often solves the issue. If it still doesn't, please send a bug report through Special:Contact!