This is based on a Check_MK plugin downloaded from the Github Repo below.

Firstly, clone the repository from Github and browse to the directory (On your Check_MK Server):

git clone https://github.com/michalskalski/cmk-apcupsd.git
cd cmk-apcupsd

Now install the plugin:

check_mk -P install cmk-apcupsd-1.0.mkp

Now download the plugin from the Check_MK agents page from your Monitoring server (See here for setting up an Agent Downloads Page if you have not already done this):

cd /usr/lib/check_mk_agent/plugins/
wget http://[Monitoring-Server]/agents/plugins/apcupsd
chmod +x apcupsd

Re-inventory the server

cmk -II [server-name]

Reload Check_MK to pull the latest plugins