Articles for free, how to get them into the chart?

In my shop there are some articles which are for free (trial). The only difference between these articles an "regular" articles is the price. But my article with price = 0 do not have a button "Add to chart". What Do I have to configure to get the Add to chart button? You may have a look at www.almwiese.at/index.php/team-casa-margherita/gewuerzoel .

Martin Wolf

After checking the settings again I found it!

System / Configuration / Display / Show "Add to Chart ..."