Comodo Free Ssl Certificate - Need To Extract Private Key

Need help.

Running WHS V1 - I have installed Comodo Free SSL Certificate for a trial

I need to extract Private Key but have no Idea on how to do this have googled but not come up with nothing much that helps.

The reason for needing the private key is:-

I'm running and e-mail server Axigen, and want a SSL connection to the webmail (Its currently set up using port 443) but want the (SSL Certificate to show its signed from a trusted root)
but to do this I need to concatenate the private key file and the SSL certificate file into one file for Axigen e-mail server.

So how do I get the private key? can you provide steps for this process?

Thanks