Also note that the order of the values is area/country dependent. Here in the US its MM/DD/YYYY, where in other areas it is often DD/MM/YYYY. I also like to make the year 4 digits, to lessen the ambiguity.
Like Enst said, Microsoft Excel is an excellent tool to load up the CSV file and change the date column format. Try exporting a dummy record in Memento to see what Memento likes for date formatting.