Hi fernipascual. The first thing to check is to look in your WordPress settings to see how many posts are set to show at a time.
Go to WP Admin > Settings > Reading Settings. If it is set to 10 and you only have a total of 8 posts, the prev/next links will not display.
Try setting it to less than 8 and see if they show up.
If you do indeed have more posts, you will need to add the prev/next tags to the bottom of your single template. Something like this should do the trick…
<div class="post-nav">
<div class="post-nav-prev">
<?php previous_post_link('%link', 'Previous Post'); ?>
</div>
<div class="post-nav-nex">
<?php next_post_link('%link', 'Next Post'); ?>
</div>
</div>
More information on the previous_post_link tag can be found here: previous_post_link