You click on "start a bounty" on an eligible question.  From the [bounty FAQ on MSO](http://meta.stackoverflow.com/questions/16065/how-does-the-bounty-system-work): "A bounty can be started on a question two days after the question was asked."

You only [unanswered question](http://serverfault.com/questions/274007/how-do-you-reboot-a-server-when-you-are-not-local-to-it) is only 1 day old right now.

![bounty][1]


  [1]: https://i.sstatic.net/N9kK3.png