The email you used to register for F-Secure Mobile Security is your user name. If you don't remember what it is, you can follow the below steps to find out:
Android devices:
1. Launch F-Secure Mobile Security on your phone
2. Tap on the Menu 
button
3. Choose 'Account'
4. Your registered email address will appear at the top of the page.
iOS devices:
1. Launch F-Secure Mobile Security on your phone
2. Tap on the gear icon on the top left
4. Tap Account
If you forgot your password, please see this article.