At the very least you need to have installed Nagios or a compatible monitoring suite.
The other plugins will be run from your Nagios server. Though you can download the latest release and install it manually the easiest way is to install it using pip for Python.
If pip isn’t installed you need to install it on your monitoring server. Typically this is done using:
sudo apt-get install python-pip sudo yum -y install python-pip
It might be different on your distribution. The binary for pip is pip or python-pip.
To install pip you can one of these commands, however this doesn’t place the plugin files in your plugins directory.
sudo pip install nelmon sudo python-pip install nelmon
To install the files to the correct location use this command and change the path to match your system:
sudo pip install nelmon --install-option="--install-scripts=/usr/local/nagios/libexec"
To upgrade Nelmon you can use:
sudo pip install -U nelmon --install-option="--install-scripts=/usr/local/nagios/libexec"