Sorry if this is a ridiculously elementary question:

I am trying to create a formula that adds up all the instances of a particular field. I tried using running totals wizard, but I only want the grand total, not a line-by-line running total.

Can anyone help me? Thank you!

Is this a numeric field you are trying to total? What section are you placing the total field in?

There are two ways you can do this - one using a basic formula and then Grand Totalling that formula; the other way is using a variable (more complex).  Here is the basic formula:

If {field.fieldA} = "Instance" then 1

Insert that formula in the details section, right mouse-click on it, Insert - Grand Total.