I can search for communities in the little list box at the bottom. I have absolutely no idea how to search for posts or comments (other than using DuckDuckGo or whatever).
It's currently not possible in jerboa to search for posts, or any other content except for communities. There is a issue created on github - so I guess it will be possible in the future.
For now you have to use the search function on the website of your instance. At least on feddit.de it's working as I would expect it.
I love how FOSS often has this glimmer of hope. With Lemmy becoming more popular, an increasing number of developers might come on board and the glimmer will start becoming a light. 😊