To get that button on your projects, you must upload your project with source. This allows other users to (1) download and save the scenarios on their own systems by pressing the 'Save' option, or (2) open the project in their temp folder using Greenfoot by pressing 'Open'. Either way, they can inspect the code and/or run the scenarios.

first ( to smandoli) my name is winkleDEN not winkleTON. (for danpost) I uploaded the space invaders project at a puplic CPU at a university, and I dont have access to the cpu lab until saturday, but i need to work on it for an assignment, but i cant download the project to my desktop.

winkleden - if you didn't tick the "publish source code" checkbox when you uploaded the scenario, then the source code didn't get uploaded and in that case there's no way you can open the scenario in Greenfoot.