This button can be used to create a new object in the currently selected container. The creation process is done with the help of the Create Object dialog:
You have to set the name and the object class of the new object. If the schema of the current LDAP server was evaluated correctly, then you find all possible object classes in the drop down menu and you get a list of all the attributes which can be set for the selected object class. Even more: The attributes which are colored blue are mandatory attributes and have to be set to create the object.
You can get detailed information about using this dialog in the manual topic Creating Directory Objects.
To create a new object, you could also use the menu option Edit - New Object..., or you just press INSERT if the mouse focus is set on the treeview panel or the object list panel.
There may be situations where the Create new object button may be disabled. This might be if the LEX ReadOnly Mode is active, or when the application focus lays on the attribute list.