what do I open it with
Its an standalone exe. It shouldn't need anouther program. Just open the folder and click the exe file.
But as a zip file you do need to uncompress it. Winrar can be used for free. And you need some kind of zip file program. Windows do not do it well enough for you.
This game is also under going a major revitalizion process. Come and join my discord if you want to keep up with it.