go to an entry page, below the game link you’ll find categories to rate. rate them and submit.

That means you have to have submitted a game in either the Compo or Jam to be allowed to vote on other entries. This is done, I’m guessing, so that people can’t have their friends and fans come and vote for their games to get to the top, or so that bots can’t be created to vote, various reasons like that. So only those who have entered games can vote, and only once on each game.