Please do not edit any of the apache config files manually, if you did any changes already, undo them as tehy will prevent the ssl website to work later. The procedure to install a ssl certificate in a website is:
1) Select the IP address in the site settings instead of *. If the IP does not show up, add it under System > Server IP.
2) Enable the ssl checkbox in the site settings.
3) Create a ssl certificate on the ssl certificate tab. If you have already created a cert that does not work, then delete this cert by selecting delete as action and press on save before you create a new ssl cert. Now test that the ssl site works with the self signed ssl cert.
4) If you want to use a signed ssl cert, then use the csr that ispconfig shows in the first field. Dont use any other csr as the crt and key will not match later and the sl site will fail.