Change Discount Code Text

I'm trying to change the text displayed for "Discount Code."

I edited C:\inetpub\EnergyDynamics\Presentation\Nop.Web\App_Data\defaultResources.public.nopres.xml
and changed the xml file as follows:

    <LocaleResource Name="DiscountCouponCode">
        <Value>Promotional Code</Value>
        <Children>
          <LocaleResource Name="Button">
            <Value>Apply promotional code</Value>
          </LocaleResource>
          <LocaleResource Name="Tooltip">
            <Value>Enter your promotional code here</Value>
          </LocaleResource>
          <LocaleResource Name="Applied">
            <Value>The promotional code was applied</Value>
          </LocaleResource>
          <LocaleResource Name="WrongDiscount">
            <Value>The promotional code you entered couldn't be applied to your order</Value>
          </LocaleResource>
          <LocaleResource Name="CurrentCode">
            <Value>Entered promotional code - {0}</Value>
          </LocaleResource>

However, the shopping cart still shows text of "Discount Code" and button shows text of "Apply coupon."

What am I missing?

I am also having the same problem the only difference is that I am added a new button "Cancel" where a customer can look up an order and cancel it if it hasn't been shipped. I did the same thing as JoeRenoylds but its still not changing the text on the button...Did anybody come with a solution for this problem??

JoeReynolds wrote:
This file is using only during installation. If you want to change resources of the already installed application, then go to admin area, configuration > languages > 'View string resources link.