Your pasted configuration doesn't have out_file or similar plugin.
Does the configuration under conf.d/* have such plugins?
BTW, I tried your configurations on my one Mac and ping_message_checker emits events correctly.
So your configuration seems correct.
2014-06-25 11:55:29 +0900 [info]: starting fluentd-0.10.50
2014-06-25 11:55:29 +0900 [info]: reading config file path="a.conf"
2014-06-25 11:55:29 +0900 [info]: gem 'fluent-mixin-config-placeholders' version '0.2.4'
2014-06-25 11:55:29 +0900 [info]: gem 'fluent-plugin-eval-filter' version '0.0.5'
2014-06-25 11:55:29 +0900 [info]: gem 'fluent-plugin-ping-message' version '0.1.1'
2014-06-25 11:55:29 +0900 [info]: gem 'fluentd' version '0.10.46'
2014-06-25 11:55:29 +0900 [info]: using configuration file: <ROOT>
<match missing.**> # changed from debug to missing for ping_message_checker
check_interval 20 # changed to short interval for testing
notification_times 2
</match>
</ROOT>
2014-06-25 11:55:29 +0900 [info]: adding source type="debug_agent"
2014-06-25 11:55:29 +0900 [info]: adding source type="forward"
2014-06-25 11:55:29 +0900 [info]: adding match pattern="missing.**" type="stdout"
2014-06-25 11:55:29 +0900 [info]: adding match pattern="ping.**" type="ping_message_checker"
2014-06-25 11:55:29 +0900 [info]: listening dRuby uri="druby://
127.0.0.1:24230" object="Engine"
2014-06-25 11:55:29 +0900 [info]: listening fluent socket on
0.0.0.0:24224
2014-06-25 11:56:49 +0900 missing.ping: {"data":"ping message from MacBook-Pro.local"}
2014-06-25 11:57:09 +0900 missing.ping: {"data":"ping message from MacBook-Pro.local"}
2014-06-25 11:57:43 +0900 [info]: shutting down fluentd
2014-06-25 11:57:43 +0900 [info]: process finished code=0
From above log, after passed 20 seconds, ping_message_checker emits events to 'missing.*' stdout.
For testing, could you try
<match ping.**>
type stdout
</match>
instead of ping_message_checker on A node?
Your configuration, check_interval 60, needs 60 seconds interval.