IMPORTANT: Make sure you are using the right plugin.
The two plugins have similar sounding, but different, function names. If you mistakenly install the wrong plugin, you will get a “call to undefined function” error. If you want to create next/previous links for your posts (including custom post types), please check out:
https://wordpress.org/extend/plugins/ambrosite-nextprevious-post-link-plus/
This plugin creates two new template tags — next_page_link_plus and previous_page_link_plus — which may be used to generate next/previous navigation links for pages. The new tags include the following options:
Extensive documentation on configuring the plugin may be found here:
http://www.ambrosite.com/plugins/next-previous-page-link-plus-for-wordpress