First of all, the programmer did not had much WordPress Knowledge...
First: DONT install it automatically via your WordPress with search and Install. Download the Plugin to your computer, open the ZIP file. there you will find another Zip File.
Upload this ZIP file to your WordPress and there is no "Wrong Headers" message.
2nd: The Smarttag [piecemaker id="1"] is NOT working at Widget areas! This is because of the programming style of this plugin. Bad!
3rd: If you get an error because of mkdir(), go to your wp-include folger and create 2 new folders:
And add then to this 2 Folders 777 Rights.
4th: Many Debug Informations are visible, ignore them. Plugin is working then.
I asked the programmer to his permission to redisign the plugin that it will work. Of corse, I dont need to have the permission for a FORK, but I would like to update it in the same SVN directory if possible.
Keep you updated.