have an error with appointments turned on...

have an error with appointments turned on, I am not really sure what it means but it shows up on a page that i have blog post on, its not a wordpress page, you can see here:

http://voice-lessons.com/index.php

also the logo disappears here: http://voice-lessons.com/voicelessons/store/products/appointment/

Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at /home/content/m/r/c/mrclmind/html/index.php:24) in /home/content/m/r/c/mrclmind/html/voicelessons/wp-content/plugins/appointments/appointments.php on line 172