How do I trust "Thawte SSL CA" security certificate?

I am using OSX Yosemite Version 10.10.5. I had to add a citrix receiver for mac. That is done. Now when I log into the citrix receiver, I am trying to access one of the programs. I keep getting a message stating, "You have not chosen to trust "Thawte SSL CA", the issuer of the server's security certificate.