The WP-PageNavi plugin requires that you add the wp_pagenavi();
function in the theme, which will require editing the theme’s files (not recommended), or creating your own function and then hooking it to the appropriate action hook (recommended, but not as easy to do).
I’ve added support for WP-PageNavi with the theme, which will be available in the next update. The theme would automatically switch to using the page navigations generated by WP-PageNavi if the plugin is installed and activated.