Unable to install Neatline - error message

I successfully set up an Omeka site, but once I attempt to install Neatline, everything disappears and I'm left with the below error message. Any idea what I might do to move forward? The instructions on troubleshooting 404 errors seem incomplete. I'm especially interested to know the answers to the "How? Where?" parenthetical regarding creating a php file in recommendation #2.

Error message:

Notice: Use of undefined constant HELPERS - assumed 'HELPERS' in /home/content/36/10494636/html/omeka/plugins/Neatline/plugin.php on line 47

Warning: require_once(HELPERS) [function.require-once]: failed to open stream: No such file or directory in /home/content/36/10494636/html/omeka/plugins/Neatline/plugin.php on line 47

Fatal error: require_once() [function.require]: Failed opening required 'HELPERS' (include_path='/home/content/36/10494636/html/omeka/application/libraries:/home/content/36/10494636/html/omeka/application/models:.:/usr/local/php5_3/lib/php:/home/content/36/10494636/html/omeka/plugins/Neatline/models') in /home/content/36/10494636/html/omeka/plugins/Neatline/plugin.php on line 47

What version of Omeka are you using?

If you're using Omeka 2.0, you should know that pretty much all plugins that don't list 2.0 as their "target version" don't work with 2.0 at the moment.