I did the following in my Header.php file. you can style according to your needs. Hope this helps
<tr>
<td><?php if (!is_user_logged_in()){ ?>
<div style="text-align:right;margin-top:10px;">
<a style="font-size:16px;text-decoration:underline;" href="/wp-login.php?redirect_to=<?php echo $_SERVER['REQUEST_URI']; ?>" class="simplemodal-login">Log In</a>?
<a style="font-size:16px;text-decoration:underline;" href="/wp-login.php?action=register" class="simplemodal-register">Register</a> </div> <?php } ?></td>
</tr>
This will create two links – login and register. On click, this will open a popup where user can enter their details
