WinFlasher actually works fine for me on Win7 64-bit, providing it's run from command-line as admin (i.e., 'Run As Administrator' cmd.exe, then start flasher from the resulting prompt).

To be honest, that tutorial looks pretty out-of-date. For one thing, the Dropbox link in it now gives a 404, and it talks about using Navifirm, which, AFAIK, doesn't allow access any more. I'd suggest reading the Comprehensive Firmware Flashing Guide, stickied in the N9/N950 subforum.

At no point do you need to 'open' the firmware files - just rename them correctly, ensure they're in the correct place, then tell flasher to use them via command-line.