itch.io

Under the description, there should be a download button. Click that, and you will download a file called Game Engine Runner.rbxl . If you have not installed Roblox Studio yet, download it and then open that file. Usually it will ask you which app you want to open it with, and choose Roblox Studio. This will then start a new project which may take a bit to load. After that, you should be able to access the whole engine. 

Hope this helps!