WorldPay (formerly SecureNet) requires you to get an account with them as well as enter your SecureNet ID and Secret Key into the WP EasyCart setup. To get your SecureNet ID and Secret Key, first log into your account via the Virtual Terminal.
Your SecureNet ID is listed immediately as you login:
You then need to navigate via the settings tab, then select key management and log into your account to view the secret key. This is shown here in a series of screenshots:
This can then be entered into your payment setup in the WordPress Admin -> Settings -> Payment panel, select WorldPay from the live gateway drop down (older versions of the cart will be listed as SecureNet, this is the same exact gateway!)