You want to add a characteristic from the free characteristics to the drilldown. You want the characteristic to be drilled down between two other characteristics in the rows of the analysis grid.
In the navigation pane, use drag and drop to drag the Calendar Year/Month characteristic from the free characteristics to the rows; drop the characteristic between Distribution Channel and Product Group.
You can also drag the characteristic directly to the analysis grid and drop it between the other two characteristics in the rows.
Result
The analysis grid is updated accordingly: Calendar Year/Monthis in the drilldown and is displayed along with its data in the rows between Distribution Channel and Product Group.