If you are installing the server, you need to put in in inetd.  A line like this one should cut the mustard:

nrpep  stream  tcp     nowait  nobody    /usr/sbin/tcpd /usr/sbin/nrpep -c /usr/local/nagios/etc/nrpep.cfg

    (All one line)

    The "nrpep" part is an entry in /etc/services for the port that nrpep needs to run on.  The "default" is 8086.  I pulled it out of my ass.  If it conflicts with anything, let me know.  Otherwise, that's the default.
An example:

nrpep        8086/tcp