i have windows 10 and it asks me what application i would like to use to open this. What do you have to let the game run?
Download 7-zip (or some other software that can uncompress .rar files) and then try opening the file. Typically files on the internet are compressed (smaller file size) so its easier on the server. Then when its uncompressed you should see "LittlePolys.exe" which you should run.