After Directory Server has started, search the error log, instance-path/logs/errors, for Hello, World! You should find an entry similar to the following line, which is wrapped for the printed page:
[02/Jan/2006:16:56:07 +0100] - INFORMATION - hello() in My first plug-in - conn=-1 op=-1 msgId=-1 - Hello, World!
At this point, reset the log level to the default to avoid logging unnecessary messages:
$ dsconf set-log-prop -h localhost -p 1389 error level:default