You may receive a notification after you've received a reward that it is available on Tremendous. You don't need to create an account on Tremendous to redeem your BetaTesting rewards.
When you're ready to start the redemption process:
Log in to your BetaTesting account and go to Settings on the top right Navigation Bar.
Find the Payments section and click on Account balance.
Click the "Transfer via Tremendous" button. After that, you should receive an email from Tremendous with further instructions on redeeming your rewards. You can choose from hundreds of options supported by Tremendous, including gift cards, bank transfers, and PayPal.