Under Basic Options, DATE AND TIME, enter 2011-03-11,05:46:24 as the Start date and 2011-03-12,05:46:24 as the End date. This gives you a day of aftershocks.

Set the minimum magnitude to 2.5.

For the GEOGRAPHIC REGION, subheading Rectangle, enter 42 in the North box, 34 in the South box, 140 in the West box, and 146 in the East box.

Choose Map and List as the FORMAT and Time-Oldest First as the ORDER BY option. 

Click search.

Take note of the number of events plotted on the map. This is the number for day 1 of your Omori plot.

Redo the search changing nothing except under DATE AND TIME enter 2011-03-12,05:46:24 as the Start date and 2011-03-13,05:46:24 as the End date. This gives you the second day of aftershocks.

Click search.

Take note of the number of events plotted on the map. This is the number for day 2 of your Omori plot.

Keep going until you think you have enough data to interpret your plot.