I implore you to do the following steps:
1. Set 'Kind of Project' to Downloadable
2. Select 'Add External File' (it's to the right of the 'Upload File' and 'Choose From Dropbox' buttons)
3. Change type from Executable to Source Code ( I don't think this step really matters, but just in case)
4. Copy and paste the URL for the game into the 'http://' section