Okay, so after a few hours, I have progressed slightly: It was initially possibly a 7z compatibility issue. I downloaded the current version of "7-Zip Extra: standalone console version, 7z DLL, Plugin for Far Manager". I renamed 7za.dll and 7za.exe to 7z.dll and 7z.exe and replaced both existing files in "C:\Program Files\Unity \Editor\Data\Tools" with the newer versions. Now it decompresses the package and when I click on Playmaker 1.8.9 in the folder it takes me back to the "import unity package' pop-up - only this time I can't select anything (I assume this means it's installed) and there is no Playmaker on the top menu bar. Any help/time is appreciated.