Thanks ColinMarquardt for the hints. I didn't see the "small" red X there because Merkaartor was overlapped by another window but it works ;)
I did know about the mapstyle but MapnikPlus is so slow one can't really work with (at least on my laptop and my desktop). It's true that it displays an "i"-icon. I realised when I was working on trunks some days later on another computer who had the MapnikPlus style.
It looks I'm getting along with Merkaartor better now. Thanks again for the hints.

Hi there,
I wanted to add to your wiki but need an account there:
- Check data for errors (and correct them automatically if possible) (Things like duped nodes, self-crossing ways, node more than once in way, ...)
- Render data to images (export as PNG, JPEG, whatever)