All you need to do is enter an expression like this:

=RunningValue(Fields!NetQty.Value, Sum, "grpGrower")

• The first parameter is the expression that is to be aggregated, in this case a field in my dataset.
• The second parameter is what aggregation function to perform (it could also be something like Count, Max or Min)
• The third and last parameter determines the scope of the aggregation. If you supply Nothing, the total spans over the whole dataset. If you specify the name of a containing group (as I do above), the total restarts for each new value in that group.

See http://technet.microsoft.com/en-us/library/ms159136.aspx for more.

This is what the output looks like, the leftmost column "Received" has a running total in the rightmost column "Received *".