Find the Worldpay.dll.config file in the same folder as the Worldpay.dll you just changed. Open this in notepad for editing and add this new section into the settings, ideally between the CallbackPassword and InstallID ones. The new section looks like this:

Now, we need to add a value to this setting. Go to your Kartris back end, Configuration > Payment and Shipping Gateways and you should see Worldpay near bottom. The verson should show as 2.1.0.1 (or above). Click to edit Worldpay.

You should see the new MD5Secret field. Add a value - this should be text, between 20 and 30 characters, no spaces. It must contain at least one number, one lowercase letter, one uppercase letter and one special character. Copy this value as you'll need it in the next step.