ok, in general you will use the =Sum([fieldname]) function in the control that will show your sum. In your case, since you have a text value you can't directly sum it, you must convert it first, with for instance the CInt() function to retrieve an integer value or a CDbl() to retrieve a decimal number. Check in the guide the conversion functions.
So in your case you may want to use:
in a control you'll create in the section at the end of report.
I hope that helps.
By the way, if you are using groups in your report you can add a total to each group for a certain field, just use the same function in the group header or footer area.