Please make sure you switch to "Advanced View" before anything else (the floating right sidebar on every Zaxaa page).
First, go to:
and enter the required info for the payment processor/gateway you want to use.
Next, go to:
>> choose the product you want to do the test purchase on >> under the "Actions" column, click "Test Purchase" link (the "arrow pointing up" icon):
Note: All test purchase will not be saved as a transaction.