If your computer is a MAC then you will need to run the Windloadcalc.com program on Microsoft Excel specifically for a MAC. There are a couple different options.
Option 1: Purchase Microsoft Excel and download it to your computer.
Payment: Full amount, charge once.
Option 2: Purchase Office 365 and download it to your computer. Microsoft 365 provides the latest versions of the entire Office suite. They are are always updated to the most recent version without having to repurchase. It is cloud based and can be run from applications on your phone.
Payment: Small amount, charged monthly.
To run Windloadcalc.com from your iPhone or tablet, download this application from your iTunes Store to run Windows MS Excel on you Apple product.