Put a textbox into the Report Footer and type
=Sum([Account]) into it

or

=Sum([Account]) + Sum([Product])

if you want to add both

If you want to add up a group eg all the accounts for one client, then use
the Sorting Grouping box to Sort by that field and there, you can choose to
have a Group Footer or Header. You can copy and paste the control from the
report footer into the Group Header or Footer and it will now add up that
group.

Evi