Comments plus 1.6.1

Comments plus 1.6.1
Hi I'm sorry but the actual Plugin I want help with is not showing up in the above drop down menu due to the below problem.

I have just tried to install the above plugin on my site and it completely locked me out of any web access to the site. I managed to delete it via FTP but the moment I try and activate the plugin I'm completely locked out of any web access to it. All I can find in the error logs is this:
[11-Apr-2013 20:05:06 UTC] PHP Fatal error: Cannot redeclare class OAuthSignatureMethod_HMAC_SHA1 in "edited out" wp-content/plugins/comments-plus/lib/external/twitter/OAuth.php on line 120

Any help is appreciated thanks.