Stamp is only allowed in the Documents screen for any document which is a PDF or can be converted to PDF. In order to stamp a document it is necessary to have working integration with 3rd party software as described in the installation guide.
If you have the write permission, select a document and click on the Stamp icon of the tool bar.
A dialog box allows you to choose one of the stamps assigned to you.
Please select one and click on Apply, now the stamp is printed in the document and a new stamp icon appears on the left.
You can click on this icon to download the PDF conversion with the recently applied stamp.