The last step in the guide is to install “iPhone configuration Utility for Mac 3.5”. However this is not available on the internet anymore and when I find an older version on it, then I’m unable to install this on my mac which is running macOS Sierra, Version 10.12.6
If I can’t find that tool and use it, how will I then be able to follow the last step in the installation guide “Install Guide Certificate via USB”???
Thanks in advance 
You can use the following procedure instead:
On the AppleTV
- open AppleTV’s ‘Settings’ app
- Select ‘General’ and select ‘Network’
- set the DNS server to the network connection to the IP address of the machine where PlexConnect is installed
- Again Select ‘General’ and highlight ‘Send Data To Apple’
- briefly press ‘Play’ on the remote.
- Remove any older profile listed here
- Select 'Add Profile"
- Enter http://trailers.apple.com/trailers.cer or http://video-api-secure.wsj.com/marketwatch.cer to install certificates
Video demo of what he said: