nrpe (icinga) client service doesn't load/run
If you have already installed NRPE-OSX there is no need to create a second LaunchDaemon.

The installer creates - besides a user nagios and group nagios - a file named com.nagios.nrpe.plist in /Library/LaunchDaemons with almost the same content as yours.

In addition you probably didn't change ownership of the file to root:staff and UserName/GroupName (admin/staff) in the plist are presumably false.