For Win x64 go here:
http://ffmpeg.zeranoe.com/builds/win64/shared/ (this is the link in the first post)
- download the first 7z (7zip) file, it has 'latest' on the name
- open the file with 7zip or winrar or any other program that works
- open the bin folder and extract or drag the .dll files to the directory where you have reaper.exe (Usually this is "c:\program files\reaper\"). Don't copy the .exe files, there's no need for them.
Don't forget to install the K-Lite 64-bit codec pack (link in the first post), as Airon says, for all other format other than quicktime, OS decoding works better (to work with these codecs go to the "video/REX/Misc" tab in Preferences and check "Use OS video decoding instead of FFmpeg when possible").
I normally install both the x64 and x86 packs, this way you have all the codecs you may need for both 64 and 32 bit applications you might have on your computer.