This main 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).
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You can also press the PLUS key on your keyboard or use the menu option Edit - Add Array Member to create new values for multi valued attributes.
By the way: If you see the Add attribute element to an array button disabled and gray, then your LEX is probably in Read Only mode.