Some tips to help get it working.
Make sure your server has cURL installed or it may not work correctly.
After installing and you enter your RSS feed location. Go to the forum where you selected the RSS feed come from and make a fast post in there.
vBulletin Scheduled tasks don't seem to run every minute for some reason, so it may take a few minutes for it to update to twitter
I just uninstalled on my forums, and reinstalled following the instructions and it still works okay:
http://twitter.com/healthforum Feel free to register an account at health forums and make a test post to see.