1. Click the file (PKCS12 format) that contains the key and the certificate.

    explanatory screenshot to the previous description
    Screenshot certificate

  2. Select "login" for keychain and click "Add".

    explanatory screenshot to the previous description
    Screenshot macOS: installation window

  3. When prompted for the certificate password, enter the password and click "OK".

    explanatory screenshot to the previous description
    Screenshot macOS: password request

  4. After the installation you will be redirected to the Keychain Access. There you can see the certificate you just installed under "My Certificates".

    explanatory screenshot to the previous description with marker on the certificate
    Screenshot macOS: Keychain Access