Calculating Grand Total in Reports

[lil lam]

Hi,
I have a report. In the report, there is a field Current Market Value
(CMV), and I am trying to create a grand total for it at the header
or
footer of the report.
I tried what was in the book:
-create a text box, and now i see 2 boxes with the following text in
it: Text#: and Unbound
-go to properties
-go to data tab
-enter formula in control source ---> =sum([field])
-change running sum to overall or group over

This hasnt been successful for me.
I keep seeing #ERROR

Can someone help?
It should be real simple, i dont know why its not working.
Can someone help?