The Turnitin Moodle plugin is not fully compatible with Moodle 1.9 at the moment.
It works for the most part except for the tie-ins to the Moodle Gradebook, since the changes to the Moodle gradebook were pretty great from 1.8 to 1.9.
We haven't been able to work on making it compatible with 1.9 yet, but if that's ok for you, the rest of the integration should work fine.
You should be able to download the plugin from your administrator account in Turnitin.
1. Login in to Turnitin at www.turnitin.com (make sure that you are in as the administrator user type)
2. Click on green update button for the account that has been activated to use the Moodle integration
3. Click on the "add/modify API integration information" link near the bottom of the page
4. Click on the link to download the appropriate plugin for your version.
Hope this helps.