If you want to send out validation emails to all your users who have not yet validated their email addresses en masse, you can do so from the "Account" tab.
Once you click the Account tab in the top nav, navigate to the "Validate User Email Addresses" section of the page.
From there, you can view the users in your site who have yet to validate their email addresses, and choose to send out validation emails to each of them with one click.
To view which users who have yet to validate their email addresses, click the View link.
To send out an email to those users, simply click the Send Validation Emails button.
Once you click that button, a validation email will be sent to each of the users who are yet to validate.