URGENT: Enabled the Marketpress plugin, and now noone can login


I donwloaded the latest version of this plugin, dropped it in the plugins folder, and activated it in the usual way.

As i did this, the page refreshed to a white empty page. All requests to log in at the usual address
http://greyworld.org/wp-admin point to a blank page.

This is deeply annoying. I can only see this as a bug in the plugin at this point - nothing else was going on at that time. I am currently unable to do anything useful, including post new stories as a result of this.

Please can you let me know what I should do to restore my site?