Hi Andre, you can add a search field to a widget on the home page, but there is no simple way to add this to the header. When we designed the theme we purposefully decided to leave search out of the header on the expectation that users would put a search field in the sidebar of pages and posts. We figure on the home page you’ll present all the critical links and information so search isn’t as important there.
If you want to add a search field to the header you’ll need to do some template customization beyond the level of supoprt we can provide here. If you need professional help on this I recommend Codeable or WP Fixit, both linked over there –>