[Plugin: Meetup Widgets] OAuth class missing when RSVP link is pressed

Hi! I installed the plugin and configured OAuth (I think…). When I press the RSVP link, a pop-up opens with the following contents:

Fatal error: Class ‘OAuth’ not found in /home/mises768/public_html/wp-content/plugins/meetup-widgets/vs-oauth-event.php on line 47

Any hints?

http://wordpress.org/extend/plugins/meetup-widgets/