The email you used to register for Lookout is your username. If you don't remember what it is, you can follow the below steps to find out:
Android devices:
1. Launch Lookout on your phone
2. Click on your phone's Menu 
button
3. Choose 'Account'
4. Your registered email address will appear at the top of the page.
iOS devices:
1. Launch Lookout on your phone
2. Click on the 8-pointed gear on the top right
3. Scroll down to the 'About Lookout' section
4. Account name is right below the 'Terms of Service' section
If you forgot your password, please see this article.