The upvote for comments is in the lower left of the comment. The upvote for posts is harder to find: It's at the bottom left of the post, above the text box for commenting.

Also, there could be a rule where only accounts with positive karma (ie, not brand-new accounts) can upvote. I'm not sure.

(Slow response because i am also learning site features: Didn't see the 'letter' icon under my karma score.)