This attribute window button can add an additional array element to an existing multi valued attribute. It is only active if you select only one single attribute in the list and if your attribute can be multi valued (this information is evaluated from the directory schema).
You can also press the PLUS key on your keyboard or use the menu option Edit - Add Array Member.
The Add attribute element to an array button is not available if the attribute window is in text output mode. If you see the button, but it is disabled and gray, then your LEX is probably in Read Only mode - or you just marked a attribute line which is not a multivalue.