Hi Mike,
It needs to be installed on both the agent and server that you wish to run the ping command. What this plugin does is uses the systems "ping" command to ping a remote host. It can be internal to your network or external just as long as it is reachable from the agent doing the pinging.
You can use it to monitor agentless systems, network devices, or an additional mechanism to monitor for host availability/reliability.
The HQ server doesn't have a resource id. It is just the server. The resource id needs to be of the agent that you wish to issue the ping command.