Friends Plugin: Call to Undefined Function?

Huh!

Not sure what happened here, but it seems like I followed the directions and the Friends plugin isn't working. I'm getting an error message when I attempt to access the Friends menu item:

Fatal error: Call to undefined function add_action() in /. . . /home/wp-admin/friends.php on line 23

Any ideas? How could add_action() not be defined? Everything else works fine. . .