Please check the below tutorial before asking this. It has been updated.
www.joomlapolis.com/documentation/279-community-builder/tutorials/18358-using-cb-triggers
That triggers is as follows as noted in the above tutorial.
Code:
$_PLUGINS->trigger( 'activity_onAfterCreateStreamComment', array( $stream, $source, $row ) );
Feel free to post if you need clarity on a trigger variable as some variables are named a little vague. In the above for example var1 is the stream, var2 is the source object (if available) of the comment, and var3 is the comment.