For Plesk version 11
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1. Log into Plesk control panel.
2. Select the desired domain from "Domains" list.
3. Click on "Open in Control Panel" option under "Hosting" section.
4. Enable the check box "SSL support" from security section.
For Plesk version 10
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1. Log into Plesk control panel.
2. Select the desired domain from "Domains" list.
3. Select "Show Advanced Operations" option from "Websites & Domains" tab.
4. Select "Website Scripting and Security" option.
5. Enable the check box "Enable SSL support" from security section.
Note: To add self signed SSL certificate please refer the following link
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http://kb.bytehouse.co.uk/content/13/60/en/steps-to-generate-self-signed-csrssl-from-Plesk.html