Click on the download link and when it completes, go to your downloads folder. This will vary depending on what version of Windows, and browser software.
Right click "orca_setup.zip" and click "Extract All..."
In the window that appears make sure the checkbox is checked for "Show extracted files when complete"
In the windows that appears double click "ORCA1.300.exe" (You should have version 1.300 or higher)
Double click "orca_setup.zip"
In the window that opens double click "ORCA1.300.exe" (You should have version 1.300 or higher)
In all versions of Windows:
A pop-up window will appear asking if you want to allow the program. Click "yes."