You can add post navigation links to custom post types in Genesis WordPress themes by using an existing function and just extending it. That function is genesis_prev_next_post_nav and is found in genesis/lib/structure/post.php
The function just refers to posts in the code but you can extend that by adding your own custom post types to an array which then add the navigation links to the posts that belong to the same custom post type.
So in your functions.php you can add a code snippet
Just add your own custom post type name, if you have just one custom post type that you want to have navigation links for, say only portfolio and not regular posts, you can just drop the array and add in…
The navigation links will appear in the footer of the post as we have added the function to be hooked into genesis_entry_footer
Navigation Previous/Next Text
By default the titles of the post are used as the linked text but you can change these – look at the code below and at the previous_post_link and next_post_link functions.