I see that I've received a rating on my game, and that some other games have as well, but I can't figure out where to go about leaving ratings on other folks' games.

Clicking the "Rate for Extra Credits Game Jam #6" button just kicks me back to a game's submission page, with no rate button to be found there.

True!

I tried it and there is no "leave-rating-box" at the usual place

Maybe they disabled participant-rating for the jam altogether?

The rating for this jam is technically hidden as we don't have this as a rated jam, but I don't think that technical "disables" it. But all in all I'd recommend just leaving comments and feedback instead!