Hello biiiiioshock
You can just put the iTween folder in your plugins folder in your project or in the engine folder. I didn’t stop including the installer for any particular reason, I just forgot after a few months without an engine upgrade 
But it’s not necessary, manual installation does the same thing.
's a tutorial for UnrealCV (Install UnrealCV Plugin — UnrealCV 1.0.1 documentation) but it’ll work the same for iTween.