You can edit the file login_register via the Display->Views menu and add the code
Code:
<tr>
<td class="key">
<label id="confirm_emailmsg" for="confirm_register_email">Confirm E-mail:</label>
</td>
<td>
<input type="text" id="confirm_register_email" name="data[confirm][email]" value="" class="inputbox required validate-email" maxlength="100" onchange="email = document.getElementById('register_email'); if(this.value!=email.value){ alert('The confirmation email is different from the email'); this.value= ''; }" /> *
</td>
</tr>
below the email field.
