Extract the downloaded .rar or .zip file and double-click setup.exe . Follow the on-screen prompts and click "OK" or "Install" when prompted.
At its core, a driver is a translation software. The Redsail RS720C is a complex electro-mechanical device. Your computer (running Windows or Mac OS) speaks a different "language" than the plotter. The acts as a real-time interpreter.
Assuming you have lost the original CD that shipped with the machine (as most people have), here is the recommended installation path.