As far as I know there is not an out of the box option that shows you the user that is downloading the interactive report. I guess you would have to write your own logic for that.
You can create your own logging table and a dynamic action with
1) a pl/sql procedure to insert the logging into the table, and
2) submits the page with the appropriate download format, eg. f?p=&APP_ID.:&APP_PAGE_ID.:&SESSION_ID.:CSV::::
As trigger of the dynamic action you need to use either of the download buttons, so I'd suggest the jQuery selector "#apexir_dl*"
The event would be onClick of either of the download buttons (be it CSV, HTML, PDF). al of these download buttons have an id that start with "apexir_dl_" and have the file extention asd suffix (so the CSV download link would be "apexir_dl_CSV").
The PL/SQL procedure for the actual logging is accurately described in the link that Tony posted.