My older posts disappear when new posts are added – please help!!!

Thanks for the website. The website mentions the following:

“[the next/back links] is often found in a paragraph code or a division like this:

<div class=”navigation”><p><?php posts_nav_link(); ?></p></div>

The parameters of the tag are as follows:

<?php posts_nav_link(‘separator’,’prelabel’,’nextlabel’); ?>”

Where can these codes be found? Is there a possibility that these codes are missing from my site?

In which section of my theme would I need to add the code to?