to solve this need,
i add a report like this query :
SELECT prefix_user.firstname,prefix_user.lastname,prefix_user.username,prefix_user.email,FROM_UNIXTIME (prefix_user.lastaccess) AS QUAND,prefix_user.lastip FROM prefix_user order by 5 desc
The data result report is good but the data order is wrong : "order by 5 desc" is not performed.
Once the report is displayed, i have to click twice on the datetime column to sort by DESC on QUAND field.
did i make a mistake ? or is it a little bug ? Maybe the plugin does not manage ORDER BY %field position% ?