Drag and drop a table from your toolbox to your report. Drop a numeric field into a column on the footer row.
Voila! A total.
If you already have a table with a detail section, click anywhere on the table, then right click on one of the icons on the left of the rows. Select Table footer. Now drop a field in the footer for a total.
To manually add a total in a footer, use =Sum(Fields!YourField.Value).
I hope this is of some help to you.