[oelk]

If there is no variable PATH, just add it. Set the variable name to PATH and the value to the path of the ffmpeg directory (plus \bin as described in the tutorial that you mentioned, but without any semicolon) and click ok.
If you got the ffmpeg binaries from the official page and set the right path, nothing bad can happen. After you did that (you don't have to reboot), you can use ffmpeg from the command prompt.

Last word if you aren't familiar with cmd:
Please take care that the video will be saved into the current work directory, so you'll find it at your users folder (unless you specified something different).