Hi,
You can mark a column as key attribute in two ways.
1. Click on the column which you wanto mark it as key attribute after you add it to output, in properties of that column there is a key attribute property. Edit it as true.
2. Add the columns to output, go to semantics node. Here in the list of columns select the column which you wanto mark as key attribute. Now either you can click on drop down of that rows first column and select Key Attribute or you can click on key attribute button in columns pane.
Regards,
Rashmi