I am a moderater for a game, and I can't figure out how to add categories. How do you do it?

Go on the leaderboard of your game -> Click "Edit Game" -> Scroll down, at some point you'll reach "Full-Game categories" and "Individual Levels categories" -> Click on "New Category" (full-game for... full game, individual levels for... individual levels) -> Write your stuff (any% no potatoes etc etc) -> Once you're done, scroll up, and, just in case, click on the green "Save" button -> Categories should be showing in the corresponding parts of the leaderboard (full-game and / or individual levels)

Can you do it if youre not a mod?

No. And next time make a new post rather than finding a three year old one.