1. Go to Insert > Summary
  2. Select the field to summarize from the first drop down
  3. Select a calculation option (sum, distinct count, maximum, etc) from the second drop down
  4. Select which group level you would like to calculate the summary for or leave Grand Total to calculate a grand total for the whole report, in the third drop down
  5. Click OK
The summary field will display in the Group Footer or Report Footer section