Hi Andy,
Why do you need Instance icon? You are not saving one of the Test instances, you need the XML. Under <Structure> (follow the screen shot), populate the fields manually (add Values to Fields) and then click <SRC> icon (just after #) and then you would be able to see the XML. Then Cntrl + A and Cntrl + V to a Text Editor (can use Notepad). Then Save the file with <.xml> extension.
Thats it.
Regards,
Nabendu.