Our initial implementations of Sniff Live were aimed at getting regular programs compiling and running in the browser, so it would be easier for new users to write a few simple Sniff programs without having to install the system. On Windows in particular installation is a bit of work (it's MUCH easier if you're running Linux or Mac).
However the most fun projects we do with Sniff involve external hardware - usually the Arduino Uno, so getting that working with Sniff Live was something we always had as part of the plan, and now its ready for you to try out.
If you head over to live.sniff.org.uk you'll see that on the front page there's a suggestion to download the Loader app. Download and unzip it somewhere. The Loader app is the easiest way to upload intel hex files to an Uno, though if you have any other method you prefer that will work too.
Then log in as usual, and take a copy of the blink example using the "copy examples" pop up in the top left. Once you've got that, press the Arduino button in the editor to compile your code for arduino. If you were running Sniff on your own computer, this would also do the flashing for you, but unfortunately it can't because now the code is being compiled on our server, while the arduino is connected to your PC!
Once the code is compiled, press the "run" button, either in the editor, or in the sidebar (the run link might not appear straight away). This should download the hex file to your computer. The exact details of what this looks like will depend on your browser and its settings, but you should save the file, and open it.
Your PC probably doesn't know what to do with a "hex" file, so set up the file association so that it opens in the "UnoLoader.exe" that you downloaded earlier. From then on when you double click a hex file, or tell your browser to "Open" it, then it should upload the code straight to your arduino without any further intervention from you.
If UnoLoader doesn't work, then check that you have an Uno connected, and that it is appearing as a COM port. If UnoLoader does fail, then its window will stay open and it will tell you whats going wrong. Check out what it says the problem is, and if you can't figure it out, let us know.