Hey I solved it by downloading wineskin winery (not wine, which is something slightly different).
Once I booted that up I had to upgrade the engine (there's a button in the app to do it), then the create wrapper button (immediately below) went from being greyed out to being clickable.
I Created a wrapper and it told me to name it, so I gave it a name then used it to open the ss6.3 installer.
The files will go to c:/sega/mods/tw or something like that. Just let them go there for the time being and install ss6.4 using the same method. It should install to the same folder as ss6.3.
Once you've done that, if you do ctrl + click on the wrapper and 'open package contents', you can navigate to the c:/sega/mods folder (there's only a few folders in the wrapper so it's easy to find) and drag the folder ss6.3 from there into your actual mods folder.
After that it should work like any other mod.