Fatal Error

I just installed the wp greet box plugin to my wp blog and this is the message I got:

Fatal error: Call to a member function get_feed_permastruct() on a non-object in /home/golda/public_html/wp-includes/link-template.php on line 365

Now I can’t log into my blog, please help. I’m not sure what version of wp I’m using but I installed it about 3 months ago.

Golda