The file extension will be .omni
If you can't find it on your laptop download it again and watch where it goes to, will go to the place were you should be looking for it. If so delete one of them.
Setup a new folder called REW and put all REW related files in this folder, alot to easier to control.
1. Plug the mic into the laptops USB port.
2. Open REW, the program will ask
3. REW will ask if you want to use the Omnimic click yes
4. Then the program will ask to load the calibration file, click yes and then pick the .omni file and load.