To install your SSL certificate, you will need to do the following:
Login to the Plesk Control Panel.
Select Domains from the left hand menu.
Click on the domain name that the certificate is for - "yourdomain.com".
Click on the Certificates menu item.
There is a button in the middle of the page labelled Browse. Click Browse and navigate to the location of the certificate (save certificate into .cer) you received. Select it, then select Send File, this will upload and attach the certificate to the corresponding private key.
The certificate name will now appear in the list of certificates at the bottom of the page. Click on the name of the certificate in the list.
Click the Send Text button.
Now click Up Level from the top right of the screen and choose Setup.
At the top of the page, select the SSL Certificate you've installed from the SSL Certificate drop-down menu.
Click the Server item from the left hand menu.
Click on the Service Management menu item.
You now need to Stop and Start the Apache process.
Please note: Restarting Apache will not work. You must stop and start the service to complete the installation and commit the changes - your host may have to do this for you.