To test commission assignment without doing a real payment you will need to manually add a payment.
To add a payment manually, go to ~wmtsystemurl~/admin/members in the members area and click on the billing history icon for the member you want to add commission for.
In the popup click the Add Payment link.
Enter a payment amount and date. For Type and Transaction ID you can enter anything or leave blank as it doesn't affect test results.
To apply the payment visit: ~wmtsystemurl~/admin/applyaffiliatepayments.php