Apple Safari on MacOS X
In order to install Trusted Root CA certificate first check your proxy settings in Settings / Advanced as indicated on the following screenshots.
Both HTTP and HTTPS proxy setting need to correctly point to your Squid box.
Double click on the DER certificate you downloaded on the Install Trusted Certificates page and select the System chain as indicated on the following screenshot.
Confirm you are trusting this certificate by clicking on Always Trust button.
Open any HTTPS page and ensure the top certificate now comes from your Squid proxy and matches your Trusted Root CA certificate.