If you cannot switch over to the Unity Platform it is because you have already installed Unity. You need to install Unity again. When you install Unity at the start you should look for the check box that allows you to use Android.
When you have downloaded the Android SDK you should look inside the Android folder. You will see SDK manager. Right click on this and open as administrator. Then install the Android SDK packages.
If you need to create some simple sound effects for your 2D games you should take a look at BFXR.
It’s a simple online synth that you can use to create all kinds of sounds. You can even download a standalone version for offline use. It comes with lots of pre designed sounds that you can use straight away … so it’s really a great way to get some quick sound effects into your game without any fuss.
The Unity software is free and the website comes with lots of helpful tutorials. But I found it quite hard to find a real basic beginners guide to 2D Game Design… Until I found this series of video tutorials on YouTube.