Joomla! Forum - community, help and support

Hi! I'm new at mkpostman, I've installed it, but it has limited or no information at all how to integrate it with the site. Any hints would be very nice to have regarding how to add subscriptions and unsubscribe from the newsletter. Google gave me nothing, and I don't speak german. Thanks in advance.

I have a problem exporting registered users. I receive a message:
"Warning: Cannot modify header information - headers already sent by "!

What could be the problem?