The process is as following: You export the public key from the CA root certificate. This can for instance be extracted using Firefox, see attached screenshots.
    

Click on the ">"

Click on "More Information"

Here you can also see the expiry date of this certificate.

Click on "View Certificate"

Then click on "Export..."

Save the certificate.

In the CMe3100 you then go to

Configuration -> Services -> Webserver (User interface & HTTPS/FTPS settings) 
In the section "TrustStore for outgoing connections 
(Push Reports using HTTPS/FTPS)" you add the public CA root certificate 
by clicking "Add server certificate (.cer)"

Click on add server certificate (.cer)

Click on "Install"

Note: You have to do this again when the root certificate expires.