Got a blank center when install finished and also thie error when installing modules

ERROR [2] Cannot modify header information - headers already sent by (output started at /export/htdocs/opn2/html/include/opndb-errorhandler.php:53)
FOUND IN [class/class.opnsession.php]
FOUND AT 263

Did you activate the article index centerbox for displaying the articles?
Cause the articles will only displayed when you create such a centerbox or the new article centerbox.
Cause you decide the position and module for dispplaying the centerbox.
So OPN will not automatical display any content in the center area.

First of all: I'm all new to this CMS and I just downloaded and installed today as I'm testing different systems to find one that will meet my needs.

The question:
I get no content in the centerbox area no matter what I do, it's just completely blank. I made a few bogus articles, news of week etc. and try to show them there just to get a reaction - nothing.

I might not understand the logic in this system, so I hope someone will take the time and explain how to get started - I think if I get the first piece of text there I will figure out the rest.

I appreciate your help!