Install as follows:

cp cacti-nut-ups-status-0.0.1-patch/config.rb /usr/lib/ruby/1.8/
cp cacti-nut-ups-status-0.0.1-patch/nut_ups_status.rb /usr/share/cacti/site/scripts/

Add the following to your upsd.users file (/etc/nut/upsd.users)
---snip snip---
[cacti]
password=password
allowfrom=local
upsmon slave
---snip snip---

The script needs to read the users file, or it logs an error each time.

It's not exactly secure, but you can do this to resolve the problem "with a hammer"

chmod o+r /etc/nut/upsd.users

A nicer way would be the add your webserver's user to the "nut" group, so it can read it using group permissions!

If you have any issues, try doing this..

tail -f /usr/share/cacti/site/log/poller-error.log

or possibly 

tail -f /usr/share/cacti/site/log/cacti.log

Hope this helps!