You can add mandatory fields by doing the following:

1. Login as a user with at least admin privelages.
2. In the Admin menu click on User Accounts.
3. In the Module Settings menu for User Accounts click on "Manage Profile Properties"
4. In this window you can create new profile properties, specify their order and mark them as required.
5. Once done click the Apply Changes button.
6. Back at the User Accounts main module go to the Module Settings menu for User Accounts click on "User Settings"
7. Put a check next to "Require a valid Profile for Login".

That should do the trick.