Most likely, you have purchased an SSL Certificate on our website, which is why you have now received this message. After successfully validating your order, you will receive an email with a zip file attached. The .crt file contains the actual certificate, which you must install on your webserver as usual. The .ca file is the file that contains the root as well as all intermediate certificates.
Depending on the platform used, the server may require a different file extension. If this is the case, you can simply rename the .ca file extension or convert the certificate using our converter (https://secure.europeanssl.eu/en/info/converter).
Then install the .ca file in the root certificates and, if you use the IIS, restart. After that you only have to check if EuropeanSSL is really registered in the list of installed root certificates. Alternatively, you can also carry out a test using the SSL Checker on our website https://secure.europeanssl.eu/en/info/verifycrt.