Thanks for contacting us regarding this issue and thanks @Geoff_Coupe for pointing me to the new download location, I have gone ahead and updated that KB doc. Please let me know if the instructions work now!
I just tried following the updated link I posted on our tutorial page (https://kb.roonlabs.com/Roon_OS_Missing_Codecs#How_To_Add_Codecs_Step_By_Step) and the ffmpeg file there is working as expected for me. What program are you extracting the archive with? I have sometimes seen the need to use 7zip to extract it to properly work.