Click MENU and you can find it there. Just click the arrow up beside their names i.e. blocktrades, steempty, etc. or if you want to vote for bue, zappl, etc. who are not on the top 50 just type bue then zappl, etc. in the box provided and click VOTE. You can view the list of 100 Witnesses here: https://steemd.com/witnesses

Or if you want me (or someone) to vote on your behalf just type pinay and click SET PROXY :)

READ THIS: https://steemit.com/steem/@steemitguide/steemitguide-what-is-a-exactly-is-a-steem-witness-and-why-every-user-should-vote