Now I feel a bit obligated....
If you have Avisynth installed you can put plugins in the Avisynth/plugins folder and Avisynth will auto-load them each time it runs. 32 bit versions of plugins for 32 bit Avisynth and the 64 bit versions for 64 bit Avisynth. It's probably less hassle to use 32 bit versions of plugins and 32 bit Avisynth, at least for the moment, so here's 32 bit instructions.....
You need dither.dll and avstp.dll from the win32 folder in the zip file.
The included scripts also need to be in the plugins folder (dither.avis and mt_multi_expand.avis). If you open dither.avis with Notepad you'll see the GradFun3() function if you scroll down a bit, so you definitely need it. Avisynth scripts can include functions, and often they contain nothing but functions (give it a couple of days and you'll probably be experimenting with creating your own
). If a script has an avis extension and it's in the plugins folder, Avisynth will auto-load it too.
The "dither package" requires additional plugins, and there's often many versions to ensure confusion, and plugins can have dependencies of their own, but I sense you're committed and can't turn back now.....
AddGrainC needs to be in the Avisynth/plugins folder. It requires the Microsoft Visual C++ 2012 Redistributable Package. There's a link for it on the AddGrainC page in case you need to install it.
Dfttest.dll should be in the Avisynth/plugins folder. For additional funkiness, Dfttest requires libfftw3f-3.dll, but it's not an Avisynth plugin. Download fftw-3.3.5-dll32.zip from
here. libfftw3f-3.dll is inside. The 32 bit version needs to be in the Windows\SysWOW64 folder if you run a 64 bit flavour of Windows. For 32 bit Windows, it needs to be in the windows\system32 folder. There's a few plugins that require libfftw3f-3.dll, but sometimes with a different name, so make a copy of libfftw3f-3.dll and rename it fftw3.dll. Put fftw3.dll in the SysWOW64 or system32 folder as appropriate along with libfftw3f-3.dll.
RgTools is a required plugin and needs to be in the Avisynth/plugins folder. RgTools requires Microsoft Visual C++ Redistributable for Visual Studio 2017. There's a link on the RGTools page if you don't have it already.
mvtools2 is required. Download it
here. Copy the x86 version of mvtools2.dll to the Avisynth/plugins folder. Don't be afraid to do the same for Depan.dll and DepanEstimate.dll. You don't need them now but you will sooner or later.
Microsoft Visual C++ Redistributable 2015 Update 3 is a requirement for that flavour of mvtools2.
Masktools2.dll should be in the Avisynth/plugins folder too.
To be honest I'd forgotten the whole dither tools package required that much work, but I've done most of it, and all that's left is a bit of downloading for you to do.
Don't be put off. Many Avisynth functions use the above plugins and once they're working they'll keep working. No doubt you'll need additional plugins down the road (my Avisynth/plugins folder contains around 80 avsi scripts and 50+ dlls), but you'll be off to a pretty good start once this is done, and relative to getting VD's external encoder option to work, it's fairly easy to do.