The simplest way is to use the Create Samples from the About Payments2Us Tab. This will generate a DEMO FACILITY in the merchant facility tab and that will have a test account.
If you have created a Full or Partial Sandbox and that has included copies of your Merchant Facility, then you will first need go to the Merchant Facility Tab, then rename the DEMO FACILITY or delete it. Then use the Create Samples again from the Merchant Facility.
If you want to test using your payment gateway specific, vs. the Windcave generated one, or you wish to see the transactions in the Payment Gateways online portal, then you will need to contact the Payment Gateway directly and setup your own test account. Most Payment Gateway providers provide an online option for this, some require you to lodge a support request.
If in a newly generated sandbox, you may wish to review the Sandbox FAQ on Sandboxes.
TIP: If creating / generating a new new test/demo facility, you will most likely need to re-authorise Payments2Us.