Sadly Google does not give the user an easy way to add a Certificate to the list of trusted certificates. To get rid of the message you should add the certificate to your OS's trusted certificates. How this can be achieved is however system dependent.
This is the steps that should work for Ubuntu (Copied from this question, more info there):
- Click the lock icon with an x
- Choose Certificate Information
- Go to details tab
- Click on Export... (save as a file)
- Now, the following command will add the certificate (where YOUR_FILE is your exported file):
certutil -d sql:$HOME/.pki/nssdb -A -t "P,," -n YOUR_FILE -i YOUR_FILE
To list all your certificates, run the following command:
certutil -d sql:$HOME/.pki/nssdb -L
If nothing helps you can always just choose to ignore the certificate issue. Notice howerver that this means you will loose all protection from man-in-the-middle-attacks! If you still decide to solve your problem this way, having to do two extra clicks to proceed to the website every time can get quite annoying. To get around this in Chrome you can set a time in
chrome://flags/#remember-cert-error-decisions, leadings to the Error being shown only once every three months.