Try adding the game's executable file to Steam.
To do this: On Steam, click "Add a Game", then click "Add a Non-Steam Game", then click "Browse", then navigate to where you downloaded the game and double click "Juler's Mod" or "Juler's Mod.exe".
After that, go back to Steam and click "Add Selected Programs". Go to "Juler's Mod" and hit "Play".