SSL verschlüsselung und Dateien über 10 MB.

The key you need to upload is the key you can find in the client area.

This article goes in more detail as to how to install an SSL certificate (which is also linked from the main HTTPS article):

The private key and certificate can all be found if you click Install next to your issued certificate in the client area.

Actually, I just checked the status of your SSL order. It seems the certificate order is still being processed by ZeroSSL. Please wait for that to be completed first, because you can’t install an SSL certificate that hasn’t been created yet.