Here's the info from Bing on how to signup
"There are three ways to join Bing Rewards: 1.Go to bing.com/explore/rewards (using Internet Explorer 7 or later, or the latest version of Firefox, Chrome or Safari) and sign up. Visit your dashboard to find out how to start earning credits. Use the Bing Rewards credit counter, in the top-right corner of any Bing.com page, to see your available credit balance.
2.Download and install Bing Bar on your PC. Click on the Bing Rewards button to join. The Bing Rewards credit counter on the Bing Bar shows you how many credits you've earned.
3.On a mobile device, go to bing.com/explore/rewards in your default web browser and sign up. Earn credits by taking advantage of offers in your dashboard, such as using Bing to search the web on your device."