According to the official website of the developer, this application is only available for Windows OS. Until this point, the developer didn't released a version that is compatible with your Mac Pro.
To install and run Windows based applications on a Mac computer, you can use Winebottler. This application is an emulator designed for Mac OS that can run Windows based applications. You can try to run the Saral income Tax program in this way, but you might encounter compatibility issues. You can test it for yourself.