The certificate is only valid for the following names: *.opendns.com, opendns.com (Error code: ssl_error_bad_cert_domain)

trying to get to https://www.express-vpn.com while working overseas as concerned about making financial transactions over hotspot. The site is a VPN provider to encrypt my connection, so it's chicken and egg. I need the VPN to be secure but cannot be sure my connection to get to the VPN providor for setup is safe. I am using OS X Mavericks and firefox 31.0, and am trying to access express-vpn from Angola, Africa. I just transfered over from windows to MAC so if you need any troubleshooting for more info, please bear with me as I get used to this OS. Thanks.