Windows

Depending on your Windows and Internet browser security settings you see a 'Discard' button. If so, right-click to the right of the 'Discard' message and click 'Keep'.

Pc_problem

Then you should see the name of the program. Right-click again and select 'Open'.

Pc_problem2

Other versions of Windows may open 'SmartScreen', you should click 'More Options' (some versions may show an 'Actions' button then 'More Options').

Smartscreen1

Then click 'Run anyway'...

Smartscreen2

Mac OSX

Depending on your Mac OSX and Internet browser security settings the program might not start the first time. If so, press CTRL whilst double-clicking the downloaded program's icon.

Osx_problem

Then you should see a window that this time includes an 'Open' button. Click the 'Open' button.

Osx_problem2