You can try to run ps command and check similar to below:
[root@ambari1 nagios]# ps -aef|grep nagios
nagios 15065 1 0 Jan21 ? 00:06:22 /usr/bin/nagios -d /etc/nagios/nagios.cfg
From here you should be able to find where Nagios runs from and configuration location.
Also, did you receive any error during installation? If yes what was the issue? Did you pass Nagios smoke test?
Hope this helps,