In the file browser, change the file type filter to so you can see and select the .dmg file. Select your USB drive under the "Device" dropdown.
The Niresh Mavericks DMG is a modified distribution of OS X 10.9 designed to allow Mac software to run on standard Intel and AMD-based PCs—a setup commonly known as a "Hackintosh". While it was a popular entry point for many enthusiasts, it is now considered a legacy tool with significant pros and cons. niresh mavericks dmg