Hi Veronika,
The challenge requires you to give a presentation at a Mendix Meetup. The next one is at Webflight on August 28th, and as far as I can tell, the agenda is set. I don’t think you’ll be able to collect your credits on this one unfortunately.
Wieke gives you one possible option for finishing the challenge, but giving a Mendix presentation is broader than that. You can give a presentation at a company, at a school or even make a youtube video about Mendix. If in doubt, you can ask Mendix whether or not your idea counts as a presentation. When you've completed the challenge, e-mail mendix about it and they will credit your challenge.