Please follow these instructions to install your SSL certificate on ISPConfig Control Panel:
You must have activated the SSL feature (ISP Manager > ISP Reseller > Limits > checkbox SSL). If activated, the value should be 1 next to SSL on the tab Basic Data of your site.
1. Once you received your SSL certificate by e-mail, please copy and paste it into a text file and save the file with the .crt extension. (Include the tags -----BEGIN CERTIFICATE----- and -----END CERTIFICATE-----).
2. Open tab / enter basic data.
3. Go to SSL section of the site for which you need the SSL certificate.
4. Copy the content of the certificate (from step 1) and paste it in the field SSL Certificate.
5. In the field Action, click Save Certificate.
6. Click Save.