Plesk 7 & 8 Certificate Installation
You will be receiving the following four files from comodo:
Root AddTrustExternalCARoot.crt
Intermediate CA UTNAddTrustServerCA.crt
Intermediate CA PositiveSSLCA.crt
domain/site certificate yourdomainname.crt
or click to download the PositiveSSL Root and Intermediate files
Login to the Plesk 7 Control Panel.
From the left hand menu, select 'Domains'

Click on the domain name that the certificate is for

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 saved site certificate yourDOMAINNAME.crt you received from Comodo. Selecting it, then select 'Send File', this will upload and install the certificate against 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 from the list.
The box on the page labeled 'CA Certificate'. You will need to paste both the Intermediate CA certificate PositiveSSLCA.crt and UTNAddTrustServerCA.crt and Root certificates AddTrustExternalCARoot.crt from the .zip file you have received into this box.

They must be pasted this in order, the Intermediate CA certificates PositiveSSLCA.crt,UTNAddTrustServerCA.crt first and followed by the Root certificate AddTrustExternalCARoot.crt, the result will look similar to the
example below (Please note: no blank line between then end of one certificate and the start of the next):
-----BEGIN CERTIFICATE-----
MIIEyDCCBDGgAwIBAgIEAgACmzANBgkqhkiG9w0BAQUFADBFMQswCQYDVQQGEwJV
UzEYMBYGA1UEChMPR1RFIENvcnBvcmF0aW9uMRwwGgYDVQQDExNHVEUgQ3liZXJU
.....
zs1x+3QCB9xfFScIUwd21LkG6cJ3UB7KybDCRoGAAK1EqlzWINlVMr5WlvHqvaDj
vA2AOurM+5pX7XilNj1W6tHndMo0w8+xUengDA==
-----END CERTIFICATE-----
-----BEGIN CERTIFICATE-----
MIIB+jCCAWMCAgGjMA0GCSqGSIb3DQEBBAUAMEUxCzAJBgNVBAYTAlVTMRgwFgYD
VQQKEw9HVEUgQ29ycG9yYXRpb24xHDAaBgNVBAMTE0dURSBDeWJlclRydXN0IFJv
.....
IjeaY8JIILTbcuPI9tl8vrGvU9oUtCG41tWW4/5ODFlitppK+ULdjG+BqXH/9Apy
bW1EDp3zdHSo1TRJ6V6e6bR64eVaH4QwnNOfpSXY
-----END CERTIFICATE-----
-----BEGIN CERTIFICATE-----
MIIFAzCCA+ugAwIBAgIQTM1KmltFEyGMz5AviytRcTANBgkqhkiG9w0BAQUFADCB
lzELMAkGA1UEBhMCVVMxCzAJBgNVBAgTAlVUMRcwFQYDVQQHEw5TYWx0IExha2Ug
.....
zg5G8t6P2jt9HpOs/PQyKw+rAR+lQI/jJJkfXbKqDLnioeeSDJBLU30fKO5WPa8Y
Z0nf1R7CqJgrTEeDgUwuRMLvyGPui3tbMfYmYb95HLCpTqnJUHvi
-----END CERTIFICATE-----
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, change the 'SSL Certificate' drop-down menu to the certificate you have just installed.

NOTE: The example above shows a shared IP.
If there is no Certificate drop-down box on the screen, this means that you are on a shared hosting account, therefore you need to upgrade your hosting package and purchase a dedicated IP address from your hosting provider
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

NOTE: Restarting Apache will NOT work. You must stop the service, then start it again to complete the installation
NOTE:When you add a certificate, it is not installed automatically onto the domain or assigned to an IP address, but only added to the Certificate repository.