I wanted to know the same thing, but I didn't get a response.. :(
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UnityPlayer.dll is a file that should've been automatically downloaded with the game. It shows up in the folder along with the other files that should be there (Mono folder, OneHandClapping_Data folder, OneHandClapping.exe,and UnityCrashHandler64.exe.)
If it's not there, then I can only suggest redownloading the game.