you download either V1.05b_Raft_Win32.zip (download win32 only if you have 32-bit operating system) or V1.05b_ Raft_Win64.zip and extract the archive to your computer. They contain the executable and all the game files in them packed into zip archive.
Do note that this is three year old prototype of Raft and contains far less features than the commercial version on Steam.
