Previous/Next navigation w/ AJAX

I have a one-page Portfolio theme for my WP.
I´ve added “Previous” and “Next” buttons using http://codex.wordpress.org/Function_Reference/previous_post_link

To make this work in this theme I also need to add in an argument for the AJAX to load it into the same div (now it´s loading at the bottom of the page instead of the same div). I´m not an experience coder so I don´t know how to do this. I know I need to add code to the customjs.php but don´t know what to write?

Can anyone help me? Check out my web and click any project and then click “Previous” or “Next” to understand what I mean: heyhawkan.com/wordpress