If got an error "Side Loading of apps is not enabled on this site error while installing app" , while deploying an app to share point 2013 site by using VS 2013. The main reason that in share point 2013 site we have a new site template, "Developer site", We can only deploy the apps to developer site.
Finally the solution is you can just deploy the apps to developer site ( site / site collection created with developer site template.)