I picked this up, here, with the DLC after playing it via Steam from some bundle I got but I am not seeing any way to start the DLC content. As it's RenPy there's no missing saves, it's all in the engine directory and the autosaves and manual just before the festival both load fine (after the "Not from this device" warning so if you need to finish the game to start that's not an issue.

I tried starting a new game, I tried continuing from the end and finishing again to see if it gives me an option to move on to the DLC, and I tried the "Extras" section where the DLC is listed only to have it tell me I can purchase it where I purchased the game after having purchased and installed it.