1. Double click the crt file from the URL you have been given by the ISP
2. Follow the wizard and select "SYSTEM" from the destination dropbox when prompted. Your SSL Certificate (with private key and corresponding Intermediate Certificate) is now imported into your System keychain.
3. Open Keychain Access -  In the Finder window, under Favorites, click Applications, click Utilities and then double-click Keychain Access.
4. In the Category list, select system.
5. Select a certificate, then choose File > Get Info.
6. Click the Trust disclosure triangle to display the trust policies for the certificate.
7. To override the trust policies, choose new trust settings from the pop-up menus (always allow).