Unable to install PositiveSSL (Comodo) certificate

Hello,
A few days ago I purchased positiveSSL Certificate, I have done everything that is required to get keys.

Comodo sent me a email with attached keys.

Root CA Certificate - AddTrustExternalCARoot.crt

Intermediate CA Certificate - PositiveSSLCA2.crt

Your PositiveSSL Certificate - domain.com.crt

But how to activate these keys successfully.
login in the control panel, enter the domain and from there to SSL, but when I put my keys gives these errors

Invalid certificate.
Invalid key or password.
Invalid intermediate certificate

Quote from TheCry

You'd created a CSR with a Key file.
In your zip package is a folder named linux. All you need are the 2 files and your key file which you'd created before.

Hello,
Excuse me for being placed topic in wrong section.

I generate my CSR, as I used help from the following page.

I've done everything as described in the page.
Then I got Email the attached zip file.
In this zip file there are only three other file

Root CA Certificate - AddTrustExternalCARoot.crt
Intermediate CA Certificate - PositiveSSLCA2.crt
Your PositiveSSL Certificate - domain.com.crt

TheCry, thanks for the detailed description.
SSL certificate is added successfully.
But it gives me an error while trying to open the site with Adress https://mydomain.bg

Error code: ERR_SSL_PROTOCOL_ERROR
this error it displayed my browser --> Chrome