Popover plugin and WP install fatal error message

I am quite fresh to WP.

1. I installed WP (not WPMU) some time ago and have spent quite some time on this up until now.
2. Today I downloaded the popover plugin to use on the WP blog
3. I uploaded the popover plugin through the Wp admin panel - plugins - add new
4. The popover plugin was installed successfully (or so it was reported)
5. In the plugin list, I could not find the plugin (at least not one named popover), but I believe there was one named 'update notifications' that I had not seen before - so I clicked to edit this one.

Then I got the following message:
"Fatal error: Call to undefined function is_site_admin() in /var/www/wordpress/wp-content/plugins/147697289_popover-1.6/update-notifications.php on line 417"

After this, it has been impossible for me to log in or get access to the admin panel. Maybe it's an easy fix or a rookie mistake, but does anyone have any good advice on what could do/try to fix this.

Tanks,
Cato