Hey! I'm not the dev, but I can try helping you. Did you download both files before attempting to extract the game?
Sanjunghogeol_1.2.3.zip + Sanjunghogeol_1.2.3.z01
Then you right click on Sanjunghogeol_1.2.3.zip and press to extract. If you still encounter problems maybe try to install a specialized extraction tool such as WinRar