Insert a Custom Post type into my Loop

To alter your default or main loop you can add query_posts before the loop runs.

http://codex.wordpress.org/Function_Reference/query_posts
http://codex.wordpress.org/Class_Reference/WP_Query ( parameters)

For example in your above code, to include all posts including custom posts types you would write;

<?php query_posts( 'post_type=any'); ?>
<?php while (have_posts()) : the_post(); ?>
//rest of your code from above

If you want to just include all default posts and 1 custom post type called “mass_list” , I recommend using WP Query over query_posts in this case.

<?php $query = new WP_Query( array( 'post_type' => array( 'post',  'mass_list' ) ) ); ?>
<?php while (have_posts()) : the_post(); ?>
    //rest of your code from above

You can drill down by looking at the parameters of WP Query in the link above, as you can see the options of WP Query allow for a lot of custom output.