How do I download a sandbox project? I want to develop the recommendation platform for job orders. In drupal there is the Prediction IO module but it is a sandbox project. How can I download it?
As reported in Installing sandbox projects, sandbox projects don't have download links, since versions are not enabled for them. You need to use git to download the code.

Assuming you mean the PredictionIO module, that doesn't still have code, at the moment; it is reported in its Version control tab.


  No code is available for PredictionIO. File an issue or contact a maintainer if you have questions.


If there were code, you could download it with the following command.

git clone --branch master http://git.drupal.org/sandbox/mrded/2116851.git predictionio


In the predictionio directory, you would find the module code.

Update

On June 17, 2015, the maintainer answered to Code? saying:


  Hi guys,
  
  We actually stopped developing of this module.
  If you're interested, I can publish your codebase as full project and give you a maintainer permissions to fully control the module.


If nobody is going to offer to be maintainer of that sandbox project, the project will not have any code.