To resend a receipt email to a registrant go to "Registration" on the blue navigation bar inside of your event. Find the registrant you would like to email in the Registration window, use the search bar in the upper right of the window to help you. Click on the registrant's name to view their Registration Details.
In the upper right corner there is a "Resend Receipt Email" button. Clicking this will resend the confirmation email to the email listed in the Registration Details.