I made this rough draft because you seem to know the lesser details but, if you need more detailed steps let me know. I currently don't have any screen recording software installed but, I'll install some if needed.
With that, are you Windows or are you running Wine through Linux? Also, let us know the version of OS you're running. Where did you store your files? If you have them installed in the LG Flash Tool installation folder or directly on the C:\ drive, Windows security may be blocking access to the files. If that's the case, simply move the files to the Desktop or say your Documents folder.
When you plug your phone in on Download Mode, you'll hear Windows, if that's the OS you're running, make a sound indicating a device has been connected. Make sure your volume isn't muted as hearing that sound is a good way to make sure everything is connected and Windows recognizes a device has been connected. In Device Manager you should see an item called Ports (COM & LPT) in the list. Under that category you should see the LGE AndroidNet USB Serial Port (COM 41). Now what happens is, that Ports (COM & LPT) item may disappear from the list in a few seconds then come back in a few seconds. This should happen indefinitely for as long as you have the phone plugged in. This is normal behavior as the computer doesn't see the device as having a "true function" so, it continuously refreshes trying to find more information on the device. This happens every time in Windows 8.1 as it's a newer feature not fully deployed in Windows 7. So, with that, if you are running a Windows based OS older than Windows 8.1, you may need to Show Hidden Devices in Device Manager & hit refresh a few times if the Ports (COM & LPT) item disappears and doesn't reappear in the list. When you hit the start button on LG Flash Tool, you may have it at a time when Windows didn't have the Ports (COM & LPT) item listed, meaning LG Flash Tool can't work with a device Windows isn't recognizing at the moment.
Lastly, the latest USB driver for this phone is LGUnitedMobileDriver_S52MAN314AP22_ML_WHQL_Ver_3.14.1.exe located here
http://tool.xcdn.gdms.lge.com/dn/downloader.dev?fileKey=UW00120120425 so, make sure you have the latest USB driver installed. After that, you still have to use the phone to install the rest of the drivers.
1) Turn the phone off and plug it in. Drivers may need to install.
2) Turn the phone back on & turn on USB Debugging & Unknown Sources. Drivers may need to install.
3) Put the phone in download mode. Drivers may need to install.
After taking those steps, all necessary drivers should be installed. Then try the flashing process again.