You can only vote when the contest is open to voting when the entry deadline has passed. When a contest opens for voting, a "VOTE" button will appear under each Instructable picture on the contest page, and there will also be a "VOTE" button on actual Instructable page just below the contest logo. You can also "UNVOTE" if you change your mind. The Pizza Contest will be the next one to open for voting on Sept. 5, and you can see how the voting works.
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Thanks for the info Angry Redhead