If you have a Kindle Fire you will will be able to find Odd Socks in the app store - it has an "actually free" sash across the icon.
If you are using a non Amazon android device you'll need to install the Amazon Underground App first.
Go to either https://www.amazon.co.uk/underground or https://www.amazon.com/underground depending on which country you are in. Follow the on-screen instructions. Once installed you will be able to find the odd socks game with the "actually free" sashed icon.