OpenVPN is already contained in most big Linux distributions (such as Debian, Ubuntu, Fedora and OpenSUSE).
In these cases, look for OpenVPN in your software installation tool.
Otherwise, download the software from
here and install it on your Linux PC.
After installation, download the credentials. This will only be possible if your have network access to your HomeTunnel box! To do this, log into your HomeTunnel account and go to the local administration interface of your HomeTunnel box. There, you will find the credential files (for both direct and proxy mode):
You can use the downloaded .ovpn files with the OpenVPN client executable in order to open your VPN connection. You may need root privileges in order to allow this.