Sample WordPress Data Import Plugin

gistfile1.php

<?php
/**
 * Plugin Name: Run-Once Data Importer
 * Description: This plugin will add custom field data to a predefined set of posts
 * Version: 1.0
 * Author: mattbeck
 */

register_activation_hook( __FILE__, 'runonce_data_import' );

function runonce_data_import() {

    $meta_key = 'key-of-custom-field';

    // Grab data you want to add to your custom field, prepare a key/val array of post IDs and data to add.
    // The real code would depend on the original data source and how you intend to compare WordPress Post IDs to the new data.
    // Could be built in a loop, pulled from a CSV, whatever.
    $import_data[0]['post_id'] = 1;
    $import_data[0]['content'] = '<div class="someclass"><p>Some more stuff in here</p></div>';
    $import_data[1]['post_id'] = 2;
    $import_data[1]['content'] = '<div class="someclass"><p>Some other different stuff in here</p></div>';

    foreach($import_data as $data){
        add_post_meta($data['post_id'], $meta_key, $data['content'], true);
        }
    }

?>

Some more stuff in here

'; // $import_data[1]['post_id'] = 2; // $import_data[1]['content'] = '

Some other different stuff in here

'; foreach($import_data as $data){ add_post_meta($data['post_title'], $meta_key, $data['content'], true); } } ``` ?>

So I added in my data, and I'd like to filter through by post_title. I activated the plugin as pasted above, and no luck. Is there something I missed?

You can't swap in post_title like that, you need the post_id

If you have only the title you'd need to run a query to get the ID first.

http://codex.wordpress.org/Function_Reference/add_post_meta