Important:

Installation is a two step process - ensure you follow both steps listed below.

Step 1: Upload your SSL Certificate

From inside PSA, choose the domain in which you are installing the SSL Certificate.
Access the domain's SSL section by clicking on the 'certificate' button.
When a CSR (certificate signing request) is generated there are two different text sections, the RSA Private Key (which was emailed to you by Plesk) and the Certificate Request. When installing a certificate, the RSA Private Key text needs to be pasted into the block preceding the web server site certificate. Example:

-----BEGIN RSA PRIVATE KEY-----
[[ENCODED BLOCK OF TEXT]]
-----END RSA PRIVATE KEY-----
-----BEGIN CERTIFICATE-----
[[ENCODED BLOCK OF TEXT]]
-----END CERTIFICATE-----
Paste the Private Key with the Certificate text into the Enter Certificate Text:text box and press the Send Text button.

Enter SSL Certificate Text

If successful a message is returned 'Certificate Successfully Installed'.
If there are any errors the old certificate will replace the new certificate that you have just sent to the server and you will be required to enter it again.
Now click Up Level to return to the Domain Administration page.

Step 2: Uploading the Rootchain Certificate

To ensure your certificate is trusted by all browsers you need to install a rootchain certificate for the domain.

From inside PSA, choose the domain in which you are installing the SSL certificate.
Access the domain's SSL section by clicking on the 'certificate' button.
The icon next to Use rootchain certificate for this domain appears on this page.
If the icon is [ON] then the rootchain certificate will be enabled for this domain. If the icon is [X] then it is disabled.
Ensure the icon is [X] before continuing (you may need to click the ON/OFF button if the icon is set to [ON]):

Rootchain Certificate

Click the browse button and locate the Intermediate CA file (located in the zip file sent to you and also available for download from the support section of the website) you saved from your Comodo issuance email.
Then click the Send File button. This will upload your Intermediate certificate to the server.
Click the icon again to set it to the [ON] state.

Now click Up Level to return to the Domain Administration page.

Using your SSL to secure logging into your Plesk Administrator

If you are applying your certificate to the Plesk control panel (in order to secure your login) you will need to login to Plesk Administrator and select Server.
Select Certificate and complete the above instructions applying your SSL Certificate to a domain.